draft-ietf-idr-bgp4-07.txt   draft-ietf-idr-bgp4-08.txt 
Network Working Group Y. Rekhter Network Working Group Y. Rekhter
INTERNET DRAFT cisco Systems INTERNET DRAFT cisco Systems
T. Li T. Li
Juniper Networks Juniper Networks
Editors Editors
<draft-ietf-idr-bgp4-07.txt> Februrary 1998 <draft-ietf-idr-bgp4-08.txt> August 1998
A Border Gateway Protocol 4 (BGP-4) A Border Gateway Protocol 4 (BGP-4)
Status of this Memo Status of this Memo
This document, together with its companion document, "Application of This document, together with its companion document, "Application of
the Border Gateway Protocol in the Internet", define an inter- the Border Gateway Protocol in the Internet", define an inter-
autonomous system routing protocol for the Internet. This document autonomous system routing protocol for the Internet. This document
specifies an IAB standards track protocol for the Internet community, specifies an IAB standards track protocol for the Internet community,
and requests discussion and suggestions for improvements. Please and requests discussion and suggestions for improvements. Please
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documents of the Internet Engineering Task Force (IETF), its Areas, documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts. working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working Drafts as reference material or to cite them other than as a "working
draft" or "work in progress". draft" or "work in progress".
To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).
1. Acknowledgments 1. Acknowledgments
This document was originally published as RFC 1267 in October 1991, This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter. jointly authored by Kirk Lougheed and Yakov Rekhter.
RFC DRAFT August 1998
We would like to express our thanks to Guy Almes, Len Bosack, and We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version of Jeffrey C. Honig for their contributions to the earlier version of
this document. this document.
We like to explicitly thank Bob Braden for the review of the earlier We like to explicitly thank Bob Braden for the review of the earlier
version of this document as well as his constructive and valuable version of this document as well as his constructive and valuable
comments. comments.
We would also like to thank Bob Hinden, Director for Routing of the We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he Internet Engineering Steering Group, and the team of reviewers he
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The primary function of a BGP speaking system is to exchange network The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reachability information with other BGP systems. This network
reachability information includes information on the list of reachability information includes information on the list of
Autonomous Systems (ASs) that reachability information traverses. Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy connectivity from which routing loops may be pruned and some policy
decisions at the AS level may be enforced. decisions at the AS level may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless BGP-4 provides a new set of mechanisms for supporting classless
RFC DRAFT August 1998
interdomain routing. These mechanisms include support for interdomain routing. These mechanisms include support for
advertising an IP prefix and eliminates the concept of network advertising an IP prefix and eliminates the concept of network
"class" within BGP. BGP-4 also introduces mechanisms which allow "class" within BGP. BGP-4 also introduces mechanisms which allow
aggregation of routes, including aggregation of AS paths. These aggregation of routes, including aggregation of AS paths. These
changes provide support for the proposed supernetting scheme [8, 9]. changes provide support for the proposed supernetting scheme [8, 9].
To characterize the set of policy decisions that can be enforced To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that a BGP speaker advertise to using BGP, one must focus on the rule that a BGP speaker advertise to
its peers (other BGP speakers which it communicates with) in its peers (other BGP speakers which it communicates with) in
neighboring ASs only those routes that it itself uses. This rule neighboring ASs only those routes that it itself uses. This rule
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transport requirements and is present in virtually all commercial transport requirements and is present in virtually all commercial
routers and hosts. In the following descriptions the phrase routers and hosts. In the following descriptions the phrase
"transport protocol connection" can be understood to refer to a TCP "transport protocol connection" can be understood to refer to a TCP
connection. BGP uses TCP port 179 for establishing its connections. connection. BGP uses TCP port 179 for establishing its connections.
This document uses the term `Autonomous System' (AS) throughout. The This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this exterior gateway protocol to route packets to other ASs. Since this
RFC DRAFT August 1998
classic definition was developed, it has become common for a single classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent single coherent interior routing plan and presents a consistent
picture of what destinations are reachable through it. picture of what destinations are reachable through it.
The planned use of BGP in the Internet environment, including such The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the issues as topology, the interaction between BGP and IGPs, and the
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applications of this architecture are for further study. applications of this architecture are for further study.
Connections between BGP speakers of different ASs are referred to as Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the "external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. Similarly, a peer in a same AS are referred to as "internal" links. Similarly, a peer in a
different AS is referred to as an external peer, while a peer in the different AS is referred to as an external peer, while a peer in the
same AS may be described as an internal peer. Internal BGP and same AS may be described as an internal peer. Internal BGP and
external BGP are commonly abbreviated IBGP and EBGP. external BGP are commonly abbreviated IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit If a particular AS has multiple BGP speakers and is providing transit
RFC DRAFT August 1998
service for other ASs, then care must be taken to ensure a consistent service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the interior routing protocol. A routes of the AS is provided by the interior routing protocol. A
consistent view of the routes exterior to the AS can be provided by consistent view of the routes exterior to the AS can be provided by
having all BGP speakers within the AS maintain direct IBGP having all BGP speakers within the AS maintain direct IBGP
connections with each other. Alternately the interior routing connections with each other. Alternately the interior routing
protocol can pass BGP information among routers within an AS, taking protocol can pass BGP information among routers within an AS, taking
care not to lose BGP attributes that will be needed by EBGP speakers care not to lose BGP attributes that will be needed by EBGP speakers
if transit connectivity is being provided. For the purpose of if transit connectivity is being provided. For the purpose of
discussion, it is assumed that BGP information is passed within an AS discussion, it is assumed that BGP information is passed within an AS
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information base; and routes that are received from other BGP information base; and routes that are received from other BGP
speakers are present in the Adj-RIBs-In. speakers are present in the Adj-RIBs-In.
If a BGP speaker chooses to advertise the route, it may add to or If a BGP speaker chooses to advertise the route, it may add to or
modify the path attributes of the route before advertising it to a modify the path attributes of the route before advertising it to a
peer. peer.
BGP provides mechanisms by which a BGP speaker can inform its peer BGP provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is no longer available for use. that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate There are three methods by which a given BGP speaker can indicate
RFC DRAFT August 1998
that a route has been withdrawn from service: that a route has been withdrawn from service:
a) the IP prefix that expresses destinations for a previously a) the IP prefix that expresses destinations for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being in the UPDATE message, thus marking the associated route as being
no longer available for use no longer available for use
b) a replacement route with the same Network Layer Reachability b) a replacement route with the same Network Layer Reachability
Information can be advertised, or Information can be advertised, or
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In summary, the Adj-RIBs-In contain unprocessed routing information In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes speaker's Decision Process; and the Adj-RIBs-Out organize the routes
for advertisement to specific peers by means of the local speaker's for advertisement to specific peers by means of the local speaker's
UPDATE messages. UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Although the conceptual model distinguishes between Adj-RIBs-In,
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an
RFC DRAFT August 1998
implementation must maintain three separate copies of the routing implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the the information vs 1 copy with pointers) is not constrained by the
protocol. protocol.
4. Message Formats 4. Message Formats
This section describes message formats used by BGP. This section describes message formats used by BGP.
Messages are sent over a reliable transport protocol connection. A Messages are sent over a reliable transport protocol connection. A
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+ + + +
| Marker | | Marker |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type | | Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker: Marker:
RFC DRAFT August 1998
This 16-octet field contains a value that the receiver of the This 16-octet field contains a value that the receiver of the
message can predict. If the Type of the message is OPEN, or if message can predict. If the Type of the message is OPEN, or if
the OPEN message carries no Authentication Information (as an the OPEN message carries no Authentication Information (as an
Optional Parameter), then the Marker must be all ones. Optional Parameter), then the Marker must be all ones.
Otherwise, the value of the marker can be predicted by some a Otherwise, the value of the marker can be predicted by some a
computation specified as part of the authentication mechanism computation specified as part of the authentication mechanism
(which is specified as part of the Authentication Information) (which is specified as part of the Authentication Information)
used. The Marker can be used to detect loss of synchronization used. The Marker can be used to detect loss of synchronization
between a pair of BGP peers, and to authenticate incoming BGP between a pair of BGP peers, and to authenticate incoming BGP
messages. messages.
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After a transport protocol connection is established, the first After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back. acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged. messages may be exchanged.
In addition to the fixed-size BGP header, the OPEN message contains In addition to the fixed-size BGP header, the OPEN message contains
the following fields: the following fields:
RFC DRAFT August 1998
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Version | | Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System | | My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time | | Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier | | BGP Identifier |
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Hold Time and the Hold Time received in the OPEN message. The Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on the basis of the Hold implementation may reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender. KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier: BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP the sender. A given BGP speaker sets the value of its BGP
RFC DRAFT August 1998
Identifier to an IP address assigned to that BGP speaker. The Identifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer. same for every local interface and every BGP peer.
Optional Parameters Length: Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present. is zero, no Optional Parameters are present.
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This document defines the following Optional Parameters: This document defines the following Optional Parameters:
a) Authentication Information (Parameter Type 1): a) Authentication Information (Parameter Type 1):
This optional parameter may be used to authenticate a BGP This optional parameter may be used to authenticate a BGP
peer. The Parameter Value field contains a 1-octet peer. The Parameter Value field contains a 1-octet
Authentication Code followed by a variable length Authentication Code followed by a variable length
Authentication Data. Authentication Data.
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
RFC DRAFT August 1998
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Auth. Code | | Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| Authentication Data | | Authentication Data |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Code: Authentication Code:
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UPDATE messages are used to transfer routing information between BGP UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE packet can be used to construct peers. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various Autonomous a graph describing the relationships of the various Autonomous
Systems. By applying rules to be discussed, routing information Systems. By applying rules to be discussed, routing information
loops and some other anomalies may be detected and removed from loops and some other anomalies may be detected and removed from
inter-AS routing. inter-AS routing.
An UPDATE message is used to advertise a single feasible route to a An UPDATE message is used to advertise a single feasible route to a
peer, or to withdraw multiple unfeasible routes from service (see peer, or to withdraw multiple unfeasible routes from service (see
RFC DRAFT August 1998
3.1). An UPDATE message may simultaneously advertise a feasible route 3.1). An UPDATE message may simultaneously advertise a feasible route
and withdraw multiple unfeasible routes from service. The UPDATE and withdraw multiple unfeasible routes from service. The UPDATE
message always includes the fixed-size BGP header, and can optionally message always includes the fixed-size BGP header, and can optionally
include the other fields as shown below: include the other fields as shown below:
+-----------------------------------------------------+ +-----------------------------------------------------+
| Unfeasible Routes Length (2 octets) | | Unfeasible Routes Length (2 octets) |
+-----------------------------------------------------+ +-----------------------------------------------------+
| Withdrawn Routes (variable) | | Withdrawn Routes (variable) |
+-----------------------------------------------------+ +-----------------------------------------------------+
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address prefixes for the routes that are being withdrawn from address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below: form <length, prefix>, whose fields are described below:
+---------------------------+ +---------------------------+
| Length (1 octet) | | Length (1 octet) |
+---------------------------+ +---------------------------+
| Prefix (variable) | | Prefix (variable) |
+---------------------------+ +---------------------------+
RFC DRAFT August 1998
The use and the meaning of these fields are as follows: The use and the meaning of these fields are as follows:
a) Length: a) Length:
The Length field indicates the length in bits of the IP The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero matches all IP addresses (with prefix, itself, of zero
octets). octets).
b) Prefix: b) Prefix:
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Attribute Type is a two-octet field that consists of the Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code Attribute Flags octet followed by the Attribute Type Code
octet. octet.
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| | Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RFC DRAFT August 1998
The high-order bit (bit 0) of the Attribute Flags octet is the The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0). set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional is the Transitive bit. It defines whether an optional
attribute is transitive (if set to 1) or non-transitive (if set attribute is transitive (if set to 1) or non-transitive (if set
to 0). For well-known attributes, the Transitive bit must be to 0). For well-known attributes, the Transitive bit must be
set to 1. (See Section 5 for a discussion of transitive set to 1. (See Section 5 for a discussion of transitive
attributes.) attributes.)
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to 1, then the third and the fourth octets of the path to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets. attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following: Codes, their attribute values and uses are the following:
a) ORIGIN (Type Code 1): a) ORIGIN (Type Code 1):
RFC DRAFT August 1998
ORIGIN is a well-known mandatory attribute that defines the ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume origin of the path information. The data octet can assume
the following values: the following values:
Value Meaning Value Meaning
0 IGP - Network Layer Reachability Information 0 IGP - Network Layer Reachability Information
is interior to the originating AS is interior to the originating AS
1 EGP - Network Layer Reachability Information 1 EGP - Network Layer Reachability Information
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the number of ASs in the path segment value field. the number of ASs in the path segment value field.
The path segment value field contains one or more AS The path segment value field contains one or more AS
numbers, each encoded as a 2-octets long field. numbers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2. Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3): c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP This is a well-known mandatory attribute that defines the IP
RFC DRAFT August 1998
address of the border router that should be used as the next address of the border router that should be used as the next
hop to the destinations listed in the Network Layer hop to the destinations listed in the Network Layer
Reachability field of the UPDATE message. Reachability field of the UPDATE message.
Usage of this attribute is defined in 5.1.3. Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4): d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may octet non-negative integer. The value of this attribute may
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AGGREGATOR is an optional transitive attribute of length 6. AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). Usage of this attribute is described (encoded as 4 octets). Usage of this attribute is described
in 5.1.7 in 5.1.7
Network Layer Reachability Information: Network Layer Reachability Information:
This variable length field contains a list of IP address This variable length field contains a list of IP address
RFC DRAFT August 1998
prefixes. The length in octets of the Network Layer prefixes. The length in octets of the Network Layer
Reachability Information is not encoded explicitly, but can be Reachability Information is not encoded explicitly, but can be
calculated as: calculated as:
UPDATE message Length - 23 - Total Path Attributes Length - UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes Length Unfeasible Routes Length
where UPDATE message Length is the value encoded in the fixed- where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Unfeasible size BGP header, Total Path Attribute Length and Unfeasible
Routes Length are the values encoded in the variable part of Routes Length are the values encoded in the variable part of
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enough trailing bits to make the end of the field fall on an enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is octet boundary. Note that the value of the trailing bits is
irrelevant. irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Unfeasible Routes Length + 2 for the fixed header + 2 octets for the Unfeasible Routes Length + 2
octets for the Total Path Attribute Length (the value of Unfeasible octets for the Total Path Attribute Length (the value of Unfeasible
Routes Length is 0 and the value of Total Path Attribute Length is Routes Length is 0 and the value of Total Path Attribute Length is
0). 0).
RFC DRAFT August 1998
An UPDATE message can advertise at most one route, which may be An UPDATE message can advertise at most one route, which may be
described by several path attributes. All path attributes contained described by several path attributes. All path attributes contained
in a given UPDATE messages apply to the destinations carried in the in a given UPDATE messages apply to the destinations carried in the
Network Layer Reachability Information field of the UPDATE message. Network Layer Reachability Information field of the UPDATE message.
An UPDATE message can list multiple routes to be withdrawn from An UPDATE message can list multiple routes to be withdrawn from
service. Each such route is identified by its destination (expressed service. Each such route is identified by its destination (expressed
as an IP prefix), which unambiguously identifies the route in the as an IP prefix), which unambiguously identifies the route in the
context of the BGP speaker - BGP speaker connection to which it has context of the BGP speaker - BGP speaker connection to which it has
been previously been advertised. been previously been advertised.
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19 octets. 19 octets.
4.5 NOTIFICATION Message Format 4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected. A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it. The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields: contains the following fields:
RFC DRAFT August 1998
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data | | Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code: Error Code:
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(Unspecific) value is used for the Error Subcode field. (Unspecific) value is used for the Error Subcode field.
Message Header Error subcodes: Message Header Error subcodes:
1 - Connection Not Synchronized. 1 - Connection Not Synchronized.
2 - Bad Message Length. 2 - Bad Message Length.
3 - Bad Message Type. 3 - Bad Message Type.
OPEN Message Error subcodes: OPEN Message Error subcodes:
RFC DRAFT August 1998
1 - Unsupported Version Number. 1 - Unsupported Version Number.
2 - Bad Peer AS. 2 - Bad Peer AS.
3 - Bad BGP Identifier. 3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter. 4 - Unsupported Optional Parameter.
5 - Authentication Failure. 5 - Authentication Failure.
6 - Unacceptable Hold Time. 6 - Unacceptable Hold Time.
UPDATE Message Error subcodes: UPDATE Message Error subcodes:
1 - Malformed Attribute List. 1 - Malformed Attribute List.
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(including message header). (including message header).
5. Path Attributes 5. Path Attributes
This section discusses the path attributes of the UPDATE message. This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories: Path attributes fall into four separate categories:
1. Well-known mandatory. 1. Well-known mandatory.
2. Well-known discretionary. 2. Well-known discretionary.
RFC DRAFT August 1998
3. Optional transitive. 3. Optional transitive.
4. Optional non-transitive. 4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations. Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every Some of these attributes are mandatory and must be included in every
UPDATE message that contains NLRI. Others are discretionary and may UPDATE message that contains NLRI. Others are discretionary and may
or may not be sent in a particular UPDATE message. or may not be sent in a particular UPDATE message.
All well-known attributes must be passed along (after proper All well-known attributes must be passed along (after proper
updating, if necessary) to other BGP peers. updating, if necessary) to other BGP peers.
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by ASs in the path. by ASs in the path.
The sender of an UPDATE message should order path attributes within The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The the UPDATE message in ascending order of attribute type. The
receiver of an UPDATE message must be prepared to handle path receiver of an UPDATE message must be prepared to handle path
attributes within the UPDATE message that are out of order. attributes within the UPDATE message that are out of order.
The same attribute cannot appear more than once within the Path The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message. Attributes field of a particular UPDATE message.
RFC DRAFT August 1998
The mandatory category refers to an attribute which must be present The mandatory category refers to an attribute which must be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or protocol extension mechanism may be purely discretionary, or
discretionary, required, or disallowed in certain contexts. discretionary, required, or disallowed in certain contexts.
attribute EBGP IBGP attribute EBGP IBGP
ORIGIN mandatory mandatory ORIGIN mandatory mandatory
AS_PATH mandatory mandatory AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory NEXT_HOP mandatory mandatory
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5.1.2 AS_PATH 5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute AS_PATH is a well-known mandatory attribute. This attribute
identifies the autonomous systems through which routing information identifies the autonomous systems through which routing information
carried in this UPDATE message has passed. The components of this carried in this UPDATE message has passed. The components of this
list can be AS_SETs or AS_SEQUENCEs. list can be AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's another BGP speaker's UPDATE message, it shall modify the route's
RFC DRAFT August 1998
AS_PATH attribute based on the location of the BGP speaker to which AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent: the route will be sent:
a) When a given BGP speaker advertises the route to an internal a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker shall not modify the AS_PATH peer, the advertising speaker shall not modify the AS_PATH
attribute associated with the route. attribute associated with the route.
b) When a given BGP speaker advertises the route to an external b) When a given BGP speaker advertises the route to an external
peer, then the advertising speaker shall update the AS_PATH peer, then the advertising speaker shall update the AS_PATH
attribute as follows: attribute as follows:
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5.1.3 NEXT_HOP 5.1.3 NEXT_HOP
The NEXT_HOP path attribute defines the IP address of the border The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the destinations listed router that should be used as the next hop to the destinations listed
in the UPDATE message. When advertising a NEXT_HOP attribute to an in the UPDATE message. When advertising a NEXT_HOP attribute to an
external peer, a router may use one of its own interface addresses in external peer, a router may use one of its own interface addresses in
the NEXT_HOP attribute provided the external peer to which the route the NEXT_HOP attribute provided the external peer to which the route
is being advertised shares a common subnet with the NEXT_HOP address. is being advertised shares a common subnet with the NEXT_HOP address.
This is known as a "first party" NEXT_HOP attribute. A BGP speaker This is known as a "first party" NEXT_HOP attribute. A BGP speaker
RFC DRAFT August 1998
can advertise to an external peer an interface of any internal peer can advertise to an external peer an interface of any internal peer
router in the NEXT_HOP attribute provided the external peer to which router in the NEXT_HOP attribute provided the external peer to which
the route is being advertised shares a common subnet with the the route is being advertised shares a common subnet with the
NEXT_HOP address. This is known as a "third party" NEXT_HOP NEXT_HOP address. This is known as a "third party" NEXT_HOP
attribute. A BGP speaker can advertise any external peer router in attribute. A BGP speaker can advertise any external peer router in
the NEXT_HOP attribute provided that the IP address of this border the NEXT_HOP attribute provided that the IP address of this border
router was learned from an external peer and the external peer to router was learned from an external peer and the external peer to
which the route is being advertised shares a common subnet with the which the route is being advertised shares a common subnet with the
NEXT_HOP address. This is a second form of "third party" NEXT_HOP NEXT_HOP address. This is a second form of "third party" NEXT_HOP
attribute. attribute.
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attribute MAY be propagated over internal links to other BGP speakers attribute MAY be propagated over internal links to other BGP speakers
within the same AS. The MULTI_EXIT_DISC attribute received from a within the same AS. The MULTI_EXIT_DISC attribute received from a
neighboring AS MUST NOT be propagated to other neighboring ASs. neighboring AS MUST NOT be propagated to other neighboring ASs.
A BGP speaker MUST IMPLEMENT a mechanism based on local configuration A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a which allows the MULTI_EXIT_DISC attribute to be removed from a
route. This MAY be done either prior to or after determining the route. This MAY be done either prior to or after determining the
degree of preference of the route and performing route selection degree of preference of the route and performing route selection
(decision process phases 1 and 2). (decision process phases 1 and 2).
RFC DRAFT August 1998
An implementation MAY also (based on local configuration) alter the An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over an external value of the MULTI_EXIT_DISC attribute received over an external
link. If it does so, it shall do so prior to determining the degree link. If it does so, it shall do so prior to determining the degree
of preference of the route and performing route selection (decision of preference of the route and performing route selection (decision
process phases 1 and 2). process phases 1 and 2).
5.1.5 LOCAL_PREF 5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known mandatory attribute that SHALL be included LOCAL_PREF is a well-known mandatory attribute that SHALL be included
in all UPDATE messages that a given BGP speaker sends to the other in all UPDATE messages that a given BGP speaker sends to the other
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from the route when propagating it to other speakers. A BGP speaker from the route when propagating it to other speakers. A BGP speaker
that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT
make any NLRI of that route more specific (as defined in 9.1.4) when make any NLRI of that route more specific (as defined in 9.1.4) when
advertising this route to other BGP speakers. A BGP speaker that advertising this route to other BGP speakers. A BGP speaker that
receives a route with the ATOMIC_AGGREGATE attribute needs to be receives a route with the ATOMIC_AGGREGATE attribute needs to be
cognizant of the fact that the actual path to destinations, as cognizant of the fact that the actual path to destinations, as
specified in the NLRI of the route, while having the loop-free specified in the NLRI of the route, while having the loop-free
property, may traverse ASs that are not listed in the AS_PATH property, may traverse ASs that are not listed in the AS_PATH
attribute. attribute.
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5.1.7 AGGREGATOR 5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.4.2). A in updates which are formed by aggregation (see Section 9.2.4.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number and IP address. attribute which shall contain its own AS number and IP address.
6. BGP Error Handling. 6. BGP Error Handling.
This section describes actions to be taken when errors are detected This section describes actions to be taken when errors are detected
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Error. The Error Subcode elaborates on the specific nature of the Error. The Error Subcode elaborates on the specific nature of the
error. error.
The expected value of the Marker field of the message header is all The expected value of the Marker field of the message header is all
ones if the message type is OPEN. The expected value of the Marker ones if the message type is OPEN. The expected value of the Marker
field for all other types of BGP messages determined based on the field for all other types of BGP messages determined based on the
presence of the Authentication Information Optional Parameter in the presence of the Authentication Information Optional Parameter in the
BGP OPEN message and the actual authentication mechanism (if the BGP OPEN message and the actual authentication mechanism (if the
Authentication Information in the BGP OPEN message is present). If Authentication Information in the BGP OPEN message is present). If
the Marker field of the message header is not the expected one, then the Marker field of the message header is not the expected one, then
RFC DRAFT August 1998
a synchronization error has occurred and the Error Subcode is set to a synchronization error has occurred and the Error Subcode is set to
Connection Not Synchronized. Connection Not Synchronized.
If the Length field of the message header is less than 19 or greater If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message, UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message, then the Error Subcode is minimum length of the NOTIFICATION message, then the Error Subcode is
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protocol. protocol.
If the Hold Time field of the OPEN message is unacceptable, then the If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An Error Subcode MUST be set to Unacceptable Hold Time. An
implementation MUST reject Hold Time values of one or two seconds. implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An An implementation MAY reject any proposed Hold Time. An
implementation which accepts a Hold Time MUST use the negotiated implementation which accepts a Hold Time MUST use the negotiated
value for the Hold Time. value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically If the BGP Identifier field of the OPEN message is syntactically
RFC DRAFT August 1998
incorrect, then the Error Subcode is set to Bad BGP Identifier. incorrect, then the Error Subcode is set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents Syntactic correctness means that the BGP Identifier field represents
a valid IP host address. a valid IP host address.
If one of the Optional Parameters in the OPEN message is not If one of the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode is set to Unsupported Optional recognized, then the Error Subcode is set to Unsupported Optional
Parameters. Parameters.
If the OPEN message carries Authentication Information (as an If the OPEN message carries Authentication Information (as an
Optional Parameter), then the corresponding authentication procedure Optional Parameter), then the corresponding authentication procedure
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If any recognized attribute has Attribute Length that conflicts with If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field Error Subcode is set to Attribute Length Error. The Data field
contains the erroneous attribute (type, length and value). contains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known field contains the Attribute Type Code of the missing well-known
attribute. attribute.
RFC DRAFT August 1998
If any of the mandatory well-known attributes are not recognized, If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute. then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and The Data field contains the unrecognized attribute (type, length and
value). value).
If the ORIGIN attribute has an undefined value, then the Error If the ORIGIN attribute has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. The Data field contains Subcode is set to Invalid Origin Attribute. The Data field contains
the unrecognized attribute (type, length and value). the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the If the NEXT_HOP attribute field is syntactically incorrect, then the
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If an optional attribute is recognized, then the value of this If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error. discarded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value). The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List. the Error Subcode is set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic The NLRI field in the UPDATE message is checked for syntactic
RFC DRAFT August 1998
validity. If the field is syntactically incorrect, then the Error validity. If the field is syntactically incorrect, then the Error
Subcode is set to Invalid Network Field. Subcode is set to Invalid Network Field.
An UPDATE message that contains correct path attributes, but no NLRI, An UPDATE message that contains correct path attributes, but no NLRI,
shall be treated as a valid UPDATE message. shall be treated as a valid UPDATE message.
6.4 NOTIFICATION message error handling. 6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via message, there is unfortunately no means of reporting this error via
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with Error Code Finite State Machine Error. with Error Code Finite State Machine Error.
6.7 Cease. 6.7 Cease.
In absence of any fatal errors (that are indicated in this section), In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However, by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist. indicated by this section does exist.
RFC DRAFT August 1998
6.8 Connection collision detection. 6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a TCP If a pair of BGP speakers try simultaneously to establish a TCP
connection to each other, then two parallel connections between this connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be connection collision. Clearly, one of these connections must be
closed. closed.
Based on the value of the BGP Identifier a convention is established Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a for detecting which BGP connection is to be preserved when a
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continues to use the existing one (the one that is already in the continues to use the existing one (the one that is already in the
OpenConfirm state). OpenConfirm state).
Comparing BGP Identifiers is done by treating them as (4-octet Comparing BGP Identifiers is done by treating them as (4-octet
long) unsigned integers. long) unsigned integers.
A connection collision with an existing BGP connection that is in A connection collision with an existing BGP connection that is in
Established states causes unconditional closing of the newly Established states causes unconditional closing of the newly
created connection. Note that a connection collision cannot be created connection. Note that a connection collision cannot be
detected with connections that are in Idle, or Connect, or Active detected with connections that are in Idle, or Connect, or Active
RFC DRAFT August 1998
states. states.
Closing the BGP connection (that results from the collision Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the NOTIFICATION resolution procedure) is accomplished by sending the NOTIFICATION
message with the Error Code Cease. message with the Error Code Cease.
7. BGP Version Negotiation. 7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest multiple attempts to open a BGP connection, starting with the highest
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In this state BGP refuses all incoming BGP connections. No In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to the Start resources are allocated to the peer. In response to the Start
event (initiated by either system or operator) the local system event (initiated by either system or operator) the local system
initializes all BGP resources, starts the ConnectRetry timer, initializes all BGP resources, starts the ConnectRetry timer,
initiates a transport connection to other BGP peer, while initiates a transport connection to other BGP peer, while
listening for connection that may be initiated by the remote listening for connection that may be initiated by the remote
BGP peer, and changes its state to Connect. The exact value of BGP peer, and changes its state to Connect. The exact value of
the ConnectRetry timer is a local matter, but should be the ConnectRetry timer is a local matter, but should be
sufficiently large to allow TCP initialization. sufficiently large to allow TCP initialization.
RFC DRAFT August 1998
If a BGP speaker detects an error, it shuts down the connection If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state and changes its state to Idle. Getting out of the Idle state
requires generation of the Start event. If such an event is requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a in persistent flapping of the speaker. To avoid such a
condition it is recommended that Start events should not be condition it is recommended that Start events should not be
generated immediately for a peer that was previously generated immediately for a peer that was previously
transitioned to Idle due to an error. For a peer that was transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time previously transitioned to Idle due to an error, the time
between consecutive generation of Start events, if such events between consecutive generation of Start events, if such events
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In response to any other event (initiated by either system or In response to any other event (initiated by either system or
operator), the local system releases all BGP resources operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle. associated with this connection and changes its state to Idle.
Active state: Active state:
In this state BGP is trying to acquire a peer by initiating a In this state BGP is trying to acquire a peer by initiating a
transport protocol connection. transport protocol connection.
RFC DRAFT August 1998
If the transport protocol connection succeeds, the local system If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its Hold Timer to a large an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent. A Hold Timer value value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested. of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
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which was originally set to a large value (see above), is which was originally set to a large value (see above), is
replaced with the negotiated Hold Time value (see section 4.2). replaced with the negotiated Hold Time value (see section 4.2).
If the negotiated Hold Time value is zero, then the Hold Time If the negotiated Hold Time value is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection; System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed processing as described below.) Finally, the state is changed
to OpenConfirm. to OpenConfirm.
RFC DRAFT August 1998
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection, transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
goes into the Active state. goes into the Active state.
If the Hold Timer expires, the local system sends NOTIFICATION If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its message with error code Hold Timer Expired and changes its
state to Idle. state to Idle.
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If the local system receives a NOTIFICATION message, it changes If the local system receives a NOTIFICATION message, it changes
its state to Idle. its state to Idle.
If the KeepAlive timer expires, the local system sends a If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer. KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle. transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
RFC DRAFT August 1998
operator) the local system sends NOTIFICATION message with operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state. Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle. and changes its state to Idle.
Whenever BGP changes its state from OpenConfirm to Idle, it Whenever BGP changes its state from OpenConfirm to Idle, it
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If the KeepAlive timer expires, the local system sends a If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer. KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message, Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer, unless the negotiated Hold it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero. Time value is zero.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with operator), the local system sends a NOTIFICATION message with
RFC DRAFT August 1998
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state. Start event is ignored in the Established state.
In response to any other event, the local system sends In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle. and changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all closes the BGP (and transport-level) connection, releases all
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In. This BGP speaker shall run its Decision Process since the In. This BGP speaker shall run its Decision Process since the
previously advertised route is not longer available for use. previously advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions in the appropriate Adj-RIB-In, and the following additional actions
shall be taken: shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus new route shall replace the older route in the Adj-RIB-In, thus
RFC DRAFT August 1998
implicitly withdrawing the older route from service. The BGP speaker implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer shall run its Decision Process since the older route is no longer
available for use. available for use.
ii) If the new route is an overlapping route that is included (see ii) If the new route is an overlapping route that is included (see
9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP 9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
speaker shall run its Decision Process since the more specific route speaker shall run its Decision Process since the more specific route
has implicitly made a portion of the less specific route unavailable has implicitly made a portion of the less specific route unavailable
for use. for use.
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applying the policies in the local Policy Information Base (PIB) to applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to all peers; Process is the set of routes that will be advertised to all peers;
the selected routes will be stored in the local speaker's Adj-RIB- the selected routes will be stored in the local speaker's Adj-RIB-
Out. Out.
The selection process is formalized by defining a function that takes The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non- the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route. negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence route shall not use as its inputs any of the following: the
of other routes, the non-existence of other routes, or the path existence of other routes, the non-existence of other routes, or the
attributes of other routes. Route selection then consists of path attributes of other routes. Route selection then consists of
individual application of the degree of preference function to each individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest feasible route, followed by the choice of the one with the highest
degree of preference. degree of preference.
RFC DRAFT August 1998
The Decision Process operates on routes contained in each Adj-RIB-In, The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for: and is responsible for:
- selection of routes to be advertised to internal peers - selection of routes to be advertised to internal peers
- selection of routes to be advertised to external peers - selection of routes to be advertised to external peers
- route aggregation and route information reduction - route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each The Decision Process takes place in three distinct phases, each
triggered by a different event: triggered by a different event:
a) Phase 1 is responsible for calculating the degree of preference a) Phase 1 is responsible for calculating the degree of preference
for each route received from an external peer, and for advertising for each route received from an external peer, and MAY also
to the other internal peers the routes that have the highest advertise to all the internal peers the routes from external
degree of preference for each distinct destination. peers that have the highest degree of preference for each distinct
destination.
b) Phase 2 is invoked on completion of phase 1. It is responsible b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into distinct destination, and for installing each chosen route into
the appropriate Loc-RIB. the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each responsible for disseminating routes in the Loc-RIB to each
external peer, according to the policies contained in the PIB. external peer, according to the policies contained in the PIB.
Route aggregation and information reduction can optionally be Route aggregation and information reduction can optionally be
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The Phase 1 decision function is a separate process which completes The Phase 1 decision function is a separate process which completes
when it has no further work to do. when it has no further work to do.
The Phase 1 decision function shall lock an Adj-RIB-In prior to The Phase 1 decision function shall lock an Adj-RIB-In prior to
operating on any route contained within it, and shall unlock it after operating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it. operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference. If the route is speaker shall determine a degree of preference. If the route is
RFC DRAFT August 1998
learned from an internal peer, the value of the LOCAL_PREF attribute learned from an internal peer, the value of the LOCAL_PREF attribute
shall be taken as the degree of preference. If the route is learned shall be taken as the degree of preference. If the route is learned
from an external peer, then the degree of preference shall be from an external peer, then the degree of preference shall be
computed based on preconfigured policy information and used as the computed based on preconfigured policy information and used as the
LOCAL_PREF value in any IBGP readvertisement. The exact nature of LOCAL_PREF value in any IBGP readvertisement. The exact nature of
this policy information and the computation involved is a local this policy information and the computation involved is a local
matter. The local speaker shall then run the internal update process matter. The local speaker shall then run the internal update process
of 9.2.1 to select and advertise the most preferable route. of 9.2.1 to select and advertise the most preferable route.
9.1.2 Phase 2: Route Selection 9.1.2 Phase 2: Route Selection
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a) the highest degree of preference of any route to the same set a) the highest degree of preference of any route to the same set
of destinations, or of destinations, or
b) is the only route to that destination, or b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules c) is selected as a result of the Phase 2 tie breaking rules
specified in 9.1.2.1. specified in 9.1.2.1.
The local speaker SHALL then install that route in the Loc-RIB, The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being replacing any route to the same destination that is currently being
RFC DRAFT August 1998
held in the Loc-RIB. The local speaker MUST determine the immediate held in the Loc-RIB. The local speaker MUST determine the immediate
next hop to the address depicted by the NEXT_HOP attribute of the next hop to the address depicted by the NEXT_HOP attribute of the
selected route by performing a lookup in the IGP and selecting one of selected route by performing a lookup in the IGP and selecting one of
the possible paths in the IGP. This immediate next hop MUST be used the possible paths in the IGP. This immediate next hop MUST be used
when installing the selected route in the Loc-RIB. If the route to when installing the selected route in the Loc-RIB. If the route to
the address depicted by the NEXT_HOP attribute changes such that the the address depicted by the NEXT_HOP attribute changes such that the
immediate next hop changes, route selection should be recalculated as immediate next hop changes, route selection should be recalculated as
specified above. specified above.
Unfeasible routes shall be removed from the Loc-RIB, and Unfeasible routes shall be removed from the Loc-RIB, and
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not intended to specify any particular implementation. BGP not intended to specify any particular implementation. BGP
implementations MAY use any algorithm which produces the same results implementations MAY use any algorithm which produces the same results
as those described here. as those described here.
a) Remove from consideration routes with less-preferred a) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS. Routes which between routes learned from the same neighboring AS. Routes which
do not have the MULTI_EXIT_DISC attribute are considered to have do not have the MULTI_EXIT_DISC attribute are considered to have
the highest possible MULTI_EXIT_DISC value. the highest possible MULTI_EXIT_DISC value.
RFC DRAFT August 1998
This is also described in the following procedure: This is also described in the following procedure:
for m = all routes still under consideration for m = all routes still under consideration
for n = all routes still under consideration for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m)) if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the In the pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the highest MULTI_EXIT_DISC attribute, the function returns the highest
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NEXT_HOP attribute of the route. NEXT_HOP attribute of the route.
c) If at least one of the candidate routes was received from an c) If at least one of the candidate routes was received from an
external peer in a neighboring autonomous system, remove from external peer in a neighboring autonomous system, remove from
consideration all routes which were received from internal peers. consideration all routes which were received from internal peers.
d) Remove from consideration all routes other than the route that d) Remove from consideration all routes other than the route that
was advertised by the BGP speaker whose BGP Identifier has the was advertised by the BGP speaker whose BGP Identifier has the
lowest value. lowest value.
RFC DRAFT August 1998
9.1.3 Phase 3: Route Dissemination 9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur: 2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP b) when locally generated routes learned by means outside of BGP
have changed have changed
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AS_PATH attribute to include only its own AS number and that of the AS_PATH attribute to include only its own AS number and that of the
peer that advertised the route (such truncation requires the ORIGIN peer that advertised the route (such truncation requires the ORIGIN
attribute to be set to INCOMPLETE). In addition an implementation is attribute to be set to INCOMPLETE). In addition an implementation is
not required to pass optional or discretionary path attributes with not required to pass optional or discretionary path attributes with
such an advertisement. such an advertisement.
When the updating of the Adj-RIBs-Out and the Forwarding Information When the updating of the Adj-RIBs-Out and the Forwarding Information
Base (FIB) is complete, the local BGP speaker shall run the external Base (FIB) is complete, the local BGP speaker shall run the external
update process of 9.2.2. update process of 9.2.2.
RFC DRAFT August 1998
9.1.4 Overlapping Routes 9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching occurs when a set of destinations are identified in non-matching
multiple routes. Since BGP encodes NLRI using IP prefixes, overlap multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
will always exhibit subset relationships. A route describing a will always exhibit subset relationships. A route describing a
smaller set of destinations (a longer prefix) is said to be more smaller set of destinations (a longer prefix) is said to be more
specific than a route describing a larger set of destinations (a specific than a route describing a larger set of destinations (a
shorted prefix); similarly, a route describing a larger set of shorted prefix); similarly, a route describing a larger set of
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If both a less and a more specific route are accepted, then the If both a less and a more specific route are accepted, then the
Decision Process MUST either install both the less and the more Decision Process MUST either install both the less and the more
specific routes or it MUST aggregate the two routes and install the specific routes or it MUST aggregate the two routes and install the
aggregated route. aggregated route.
If a BGP speaker chooses to aggregate, then it MUST add If a BGP speaker chooses to aggregate, then it MUST add
ATOMIC_AGGREGATE attribute to the route. A route that carries ATOMIC_AGGREGATE attribute to the route. A route that carries
ATOMIC_AGGREGATE attribute can not be de-aggregated. That is, the ATOMIC_AGGREGATE attribute can not be de-aggregated. That is, the
NLRI of this route can not be made more specific. Forwarding along NLRI of this route can not be made more specific. Forwarding along
such a route does not guarantee that IP packets will actually such a route does not guarantee that IP packets will actually
RFC DRAFT August 1998
traverse only ASs listed in the AS_PATH attribute of the route. traverse only ASs listed in the AS_PATH attribute of the route.
9.2 Update-Send Process 9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE The Update-Send process is responsible for advertising UPDATE
messages to all peers. For example, it distributes the routes chosen messages to all peers. For example, it distributes the routes chosen
by the Decision Process to other BGP speakers which may be located in by the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system. either the same autonomous system or a neighboring autonomous system.
Rules for information exchange between BGP speakers located in Rules for information exchange between BGP speakers located in
different autonomous systems are given in 9.2.2; rules for different autonomous systems are given in 9.2.2; rules for
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WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
routes whose destinations was carried in this field (as IP prefixes). routes whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps: The speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously 1) if the corresponding feasible route had not been previously
advertised, then no further action is necessary advertised, then no further action is necessary
2) if the corresponding feasible route had been previously 2) if the corresponding feasible route had been previously
advertised, then: advertised, then:
RFC DRAFT August 1998
i) if a new route is selected for advertisement that has the i) if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the routes, then the local BGP speaker shall advertise the
replacement route replacement route
ii) if a replacement route is not available for advertisement, ii) if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the then the BGP speaker shall include the destinations of the
unfeasible route (in form of IP prefixes) in the WITHDRAWN unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message ROUTES field of an UPDATE message, and shall send this message
to each peer to whom it had previously advertised the to each peer to whom it had previously advertised the
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route with the MULTI_EXIT_DISC attribute shall be preferred to a route with the MULTI_EXIT_DISC attribute shall be preferred to a
route without the MULTI_EXIT_DISC attribute. route without the MULTI_EXIT_DISC attribute.
b) If the local system can ascertain the cost of a path to the b) If the local system can ascertain the cost of a path to the
entity depicted by the NEXT_HOP attribute of the candidate route, entity depicted by the NEXT_HOP attribute of the candidate route,
select the route with the lowest cost. select the route with the lowest cost.
c) In all other cases, select the route that was advertised by the c) In all other cases, select the route that was advertised by the
BGP speaker whose BGP Identifier has the lowest value. BGP speaker whose BGP Identifier has the lowest value.
RFC DRAFT August 1998
9.2.2 External Updates 9.2.2 External Updates
The external update process is concerned with the distribution of The external update process is concerned with the distribution of
routing information to external peers. As part of Phase 3 route routing information to external peers. As part of Phase 3 route
selection process, the BGP speaker has updated its Adj-RIBs-Out and selection process, the BGP speaker has updated its Adj-RIBs-Out and
its Forwarding Table. All newly installed routes and all newly its Forwarding Table. All newly installed routes and all newly
unfeasible routes for which there is no replacement route shall be unfeasible routes for which there is no replacement route shall be
advertised to external peers by means of UPDATE message. advertised to external peers by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed. Any routes in the Loc-RIB marked as unfeasible shall be removed.
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external peers must be separated by at least external peers must be separated by at least
MinRouteAdvertisementInterval. Clearly, this can only be achieved MinRouteAdvertisementInterval. Clearly, this can only be achieved
precisely by keeping a separate timer for each common set of precisely by keeping a separate timer for each common set of
destinations. This would be unwarranted overhead. Any technique which destinations. This would be unwarranted overhead. Any technique which
ensures that the interval between two UPDATE messages sent from a ensures that the interval between two UPDATE messages sent from a
single BGP speaker that advertise feasible routes to some common set single BGP speaker that advertise feasible routes to some common set
of destinations received from external peers will be at least of destinations received from external peers will be at least
MinRouteAdvertisementInterval, and will also ensure a constant upper MinRouteAdvertisementInterval, and will also ensure a constant upper
bound on the interval is acceptable. bound on the interval is acceptable.
RFC DRAFT August 1998
Since fast convergence is needed within an autonomous system, this Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes received from other internal procedure does not apply for routes received from other internal
peers. To avoid long-lived black holes, the procedure does not apply peers. To avoid long-lived black holes, the procedure does not apply
to the explicit withdrawal of unfeasible routes (that is, routes to the explicit withdrawal of unfeasible routes (that is, routes
whose destinations (expressed as IP prefixes) are listed in the whose destinations (expressed as IP prefixes) are listed in the
WITHDRAWN ROUTES field of an UPDATE message). WITHDRAWN ROUTES field of an UPDATE message).
This procedure does not limit the rate of route selection, but only This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval, times while awaiting the expiration of MinRouteAdvertisementInterval,
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The amount of jitter to be introduced shall be determined by The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random multiplying the base value of the appropriate timer by a random
factor which is uniformly distributed in the range from 0.75 to 1.0. factor which is uniformly distributed in the range from 0.75 to 1.0.
9.2.4 Efficient Organization of Routing Information 9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this BGP speaker may avail itself of several methods to organize this
information in an efficient manner. information in an efficient manner.
RFC DRAFT August 1998
9.2.4.1 Information Reduction 9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class. apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following that it will place in the Adj-RIBs-Out by any of the following
methods: methods:
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practice this is not likely to be a problem, since once an IP practice this is not likely to be a problem, since once an IP
packet arrives at the edge of a group of autonomous systems, the packet arrives at the edge of a group of autonomous systems, the
BGP speaker at that point is likely to have more detailed path BGP speaker at that point is likely to have more detailed path
information and can distinguish individual paths to destinations. information and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information 9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of Aggregation is the process of combining the characteristics of
several different routes in such a way that a single route can be several different routes in such a way that a single route can be
advertised. Aggregation can occur as part of the decision process advertised. Aggregation can occur as part of the decision process
RFC DRAFT August 1998
to reduce the amount of routing information that will be placed in to reduce the amount of routing information that will be placed in
the Adj-RIBs-Out. the Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information. like type and to the Network Layer Reachability Information.
Routes that have the following attributes shall not be aggregated Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical: unless the corresponding attributes of each route are identical:
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- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH - all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of routes shall appear in all of the AS_PATH in the initial set of routes
to be aggregated. to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH shall - all tuples of the type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set (they appear in at least one of the AS_PATH in the initial set (they
may appear as either AS_SET or AS_SEQUENCE types). may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated - for any tuple X of the type AS_SEQUENCE in the aggregated
RFC DRAFT August 1998
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains Y, precedes Y in each AS_PATH in the initial set which contains Y,
regardless of the type of Y. regardless of the type of Y.
- No tuple with the same value shall appear more than once in - No tuple with the same value shall appear more than once in
the aggregated AS_PATH, regardless of the tuple's type. the aggregated AS_PATH, regardless of the tuple's type.
An implementation may choose any algorithm which conforms to these An implementation may choose any algorithm which conforms to these
rules. At a minimum a conformant implementation shall be able to rules. At a minimum a conformant implementation shall be able to
perform the following algorithm that meets all of the above perform the following algorithm that meets all of the above
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aggregated should be ignored. aggregated should be ignored.
9.3 Route Selection Criteria 9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among Generally speaking, additional rules for comparing routes among
several alternatives are outside the scope of this document. There several alternatives are outside the scope of this document. There
are two exceptions: are two exceptions:
- If the local AS appears in the AS path of the new route being - If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than considered, then that new route cannot be viewed as better than
RFC DRAFT August 1998
any other route. If such a route were ever used, a routing loop any other route. If such a route were ever used, a routing loop
could result (see Section 6.3). could result (see Section 6.3).
- In order to achieve successful distributed operation, only - In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid must avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require "unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear. experience, but the principle is clear.
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1 - Idle 1 - Idle
2 - Connect 2 - Connect
3 - Active 3 - Active
4 - OpenSent 4 - OpenSent
5 - OpenConfirm 5 - OpenConfirm
6 - Established 6 - Established
BGP Events: BGP Events:
RFC DRAFT August 1998
1 - BGP Start 1 - BGP Start
2 - BGP Stop 2 - BGP Stop
3 - BGP Transport connection open 3 - BGP Transport connection open
4 - BGP Transport connection closed 4 - BGP Transport connection closed
5 - BGP Transport connection open failed 5 - BGP Transport connection open failed
6 - BGP Transport fatal error 6 - BGP Transport fatal error
7 - ConnectRetry timer expired 7 - ConnectRetry timer expired
8 - Hold Timer expired 8 - Hold Timer expired
9 - KeepAlive timer expired 9 - KeepAlive timer expired
10 - Receive OPEN message 10 - Receive OPEN message
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others Release resources none 1 others Release resources none 1
Active (3) Active (3)
1 none none 3 1 none none 3
3 Complete initialization OPEN 4 3 Complete initialization OPEN 4
Clear ConnectRetry timer Clear ConnectRetry timer
5 Close connection 3 5 Close connection 3
Restart ConnectRetry timer Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2 7 Restart ConnectRetry timer none 2
Initiate a transport connection Initiate a transport connection
RFC DRAFT August 1998
others Release resources none 1 others Release resources none 1
OpenSent(4) OpenSent(4)
1 none none 4 1 none none 4
4 Close transport connection none 3 4 Close transport connection none 3
Restart ConnectRetry timer Restart ConnectRetry timer
6 Release resources none 1 6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5 10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1 Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1 others Close transport connection NOTIFICATION 1
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Process UPDATE failed NOTIFICATION 1 Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1 13 Close transport connection 1
Release resources Release resources
others Close transport connection NOTIFICATION 1 others Close transport connection NOTIFICATION 1
Release resources Release resources
--------------------------------------------------------------------- ---------------------------------------------------------------------
The following is a condensed version of the above state transition The following is a condensed version of the above state transition
table. table.
RFC DRAFT August 1998
Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6) | (1) | (2) | (3) | (4) | (5) | (6)
|--------------------------------------------------------------- |---------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6 1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | | | | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1 2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | | | | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1 3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | | | | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1 4 | 1 | 1 | 1 | 3 | 1 | 1
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Appendix 2. Comparison with RFC1267 Appendix 2. Comparison with RFC1267
BGP-4 is capable of operating in an environment where a set of BGP-4 is capable of operating in an environment where a set of
reachable destinations may be expressed via a single IP prefix. The reachable destinations may be expressed via a single IP prefix. The
concept of network classes, or subnetting is foreign to BGP-4. To concept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9]. BGP-4 to support the proposed supernetting scheme [9].
RFC DRAFT August 1998
To simplify configuration this version introduces a new attribute, To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures. LOCAL_PREF, that facilitates route selection procedures.
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC. The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute, certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation. which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Time is now To insure that Hold Timers are symmetric, the Hold Time is now
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accommodate the TCP user interface provided by 4.3 BSD. accommodate the TCP user interface provided by 4.3 BSD.
The notion of Up/Down/Horizontal relations present in RFC1105 has The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol. been removed from the protocol.
The changes in the message format from RFC1105 are as follows: The changes in the message format from RFC1105 are as follows:
1. The Hold Time field has been removed from the BGP header and 1. The Hold Time field has been removed from the BGP header and
added to the OPEN message. added to the OPEN message.
RFC DRAFT August 1998
2. The version field has been removed from the BGP header and 2. The version field has been removed from the BGP header and
added to the OPEN message. added to the OPEN message.
3. The Link Type field has been removed from the OPEN message. 3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced with 4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message. implicit confirmation provided by the KEEPALIVE message.
5. The format of the UPDATE message has been changed 5. The format of the UPDATE message has been changed
significantly. New fields were added to the UPDATE message to significantly. New fields were added to the UPDATE message to
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with precedence set to Internetwork Control (110) value (see also with precedence set to Internetwork Control (110) value (see also
[6]). [6]).
Appendix 6. Implementation Recommendations Appendix 6. Implementation Recommendations
This section presents some implementation recommendations. This section presents some implementation recommendations.
6.1 Multiple Networks Per Message 6.1 Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same The BGP protocol allows for multiple address prefixes with the same
RFC DRAFT August 1998
AS path and next-hop gateway to be specified in one message. Making AS path and next-hop gateway to be specified in one message. Making
use of this capability is highly recommended. With one address prefix use of this capability is highly recommended. With one address prefix
per message there is a substantial increase in overhead in the per message there is a substantial increase in overhead in the
receiver. Not only does the system overhead increase due to the receiver. Not only does the system overhead increase due to the
reception of multiple messages, but the overhead of scanning the reception of multiple messages, but the overhead of scanning the
routing table for updates to BGP peers and other routing protocols routing table for updates to BGP peers and other routing protocols
(and sending the associated messages) is incurred multiple times as (and sending the associated messages) is incurred multiple times as
well. One method of building messages containing many address well. One method of building messages containing many address
prefixes per AS path and gateway from a routing table that is not prefixes per AS path and gateway from a routing table that is not
organized per AS path is to build many messages as the routing table organized per AS path is to build many messages as the routing table
skipping to change at page 56, line 25 skipping to change at page 59, line 5
6.2 Processing Messages on a Stream Protocol 6.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet possible to determine how much data has been received but not yet
processed. processed.
RFC DRAFT August 1998
One method that can be used in this situation is to first try to read One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been fill it with data as available until a complete message has been
received. received.
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6.5 Path attribute ordering 6.5 Path attribute ordering
Implementations which combine update messages as described above in Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order. 6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from This permits them to quickly identify sets of attributes from
different update messages which are semantically identical. To different update messages which are semantically identical. To
facilitate this, it is a useful optimization to order the path facilitate this, it is a useful optimization to order the path
attributes according to type code. This optimization is entirely attributes according to type code. This optimization is entirely
optional. optional.
RFC DRAFT August 1998
6.6 AS_SET sorting 6.6 AS_SET sorting
Another useful optimization that can be done to simplify this Another useful optimization that can be done to simplify this
situation is to sort the AS numbers found in an AS_SET. This situation is to sort the AS numbers found in an AS_SET. This
optimization is entirely optional. optimization is entirely optional.
6.7 Control over version negotiation 6.7 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which cannot be Since BGP-4 is capable of carrying aggregated routes which cannot be
properly represented in BGP-3, an implementation which supports BGP-4 properly represented in BGP-3, an implementation which supports BGP-4
skipping to change at page 58, line 23 skipping to change at page 61, line 4
same order if either: same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X - X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes. in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the (ASs that are between the two consecutive ASs that are the
RFC DRAFT August 1998
same) in both attributes are combined into an AS_SET path same) in both attributes are combined into an AS_SET path
segment that consists of the intervening ASs from both AS_PATH segment that consists of the intervening ASs from both AS_PATH
attributes; this segment is then placed in between the two attributes; this segment is then placed in between the two
consecutive ASs identified in (a) of the aggregated attribute. consecutive ASs identified in (a) of the aggregated attribute.
If two consecutive ASs identified in (a) immediately follow If two consecutive ASs identified in (a) immediately follow
each other in one attribute, but do not follow in another, then each other in one attribute, but do not follow in another, then
the intervening ASs of the latter are combined into an AS_SET the intervening ASs of the latter are combined into an AS_SET
path segment; this segment is then placed in between the two path segment; this segment is then placed in between the two
consecutive ASs identified in (a) of the aggregated attribute. consecutive ASs identified in (a) of the aggregated attribute.
skipping to change at page 59, line 21 skipping to change at page 62, line 5
[7] "Information Processing Systems - Telecommunications and [7] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to Inter-domain Routeing Information among Intermediate Systems to
Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993 Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993
[8] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter- [8] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993. Strategy", RFC1519, September 1993.
RFC DRAFT August 1998
[9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation [9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
with CIDR", RFC 1518, September 1993. with CIDR", RFC 1518, September 1993.
Security Considerations Security Considerations
Security issues are not discussed in this document. Security issues are not discussed in this document.
Editors' Addresses Editors' Addresses
Yakov Rekhter Yakov Rekhter
cisco Systems, Inc. cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
email: yakov@cisco.com email: yakov@cisco.com
Tony Li Tony Li
Juniper Networks, Inc. Juniper Networks, Inc.
3260 Jay St. 385 Ravendale Dr.
Santa Clara, CA 95051 Mountain View, CA 94043
(408) 327-1906 (650) 526-8006
email: tli@juniper.net email: tli@juniper.net
 End of changes. 

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