wdiff draft-ietf-idr-as-migration-02.txt draft-ietf-idr-as-migration-03.txt

Internet Engineering Task Force W. George
Internet-Draft Time Warner Cable
Intended status: Standards Track S. Amante
Expires: January 30, April 2, 2015 Apple, Inc.
July
September 29, 2014

Autonomous System (AS) Migration Features and Their Effects on the BGP
AS_PATH Attribute
draft-ietf-idr-as-migration-02
draft-ietf-idr-as-migration-03

Abstract

This draft discusses common methods of managing an ASN migration
using some BGP feaures that features for ASN migration that, while commonly-used
commonly used, are not formally part of the BGP4 protocol
specification and may be vendor-specific in exact implementation. It
is necessary to document these de facto standards to ensure that they
are properly supported in future BGP protocol work such as BGPSec.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 30, April 2, 2015.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Documentation note . . . . . . . . . . . . . . . . . . . 3
2. ASN Migration Scenario Overview . . . . . . . . . . . . . . . 4
3. External BGP Autonomous System Migration Features . . . . . . 6
3.1. Modify Inbound BGP AS_PATH Attribute . . . . . . . . . . 6
3.2. Modify Outbound BGP AS_PATH Attribute . . . . . . . . . . 8 7
3.3. Implementation . . . . . . . . . . . . . . . . . . . . . 9 8
4. Internal BGP Autonomous System Migration Features . . . . . . 10 9
4.1. Internal BGP Alias . . . . . . . . . . . . . . . . . . . 10
4.2. Implementation . . . . . . . . . . . . . . . . . . . . . 13 12
5. Additional Operational Considerations . . . . . . . . . . . . 14 13
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 14
10. Appendix: Implementation report . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 16 15
11.2. Informative References . . . . . . . . . . . . . . . . . 16 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 16

1. Introduction

This draft discusses common methods of managing an ASN migration
using some BGP features that for ASN migration that, while commonly-used
commonly used, are not formally part of the BGP4 [RFC4271] protocol
specification and may be vendor-
specific vendor-specific in exact implementation.
These features are local to a given BGP Speaker and do not require
negotiation with or cooperation of BGP neighbors. The deployment of
these features do not need to interwork with one another to
accomplish the desired results, so slight variations between existing
vendor implementations exist. exist, and will not necessarily be harmonized
due to this document. However, it is necessary to document these de
facto standards to ensure that new implementations can be successful,
and any future protocol enhancements to BGP that propose to read,
copy, manipulate or compare the AS_PATH attribute can do so without
inhibiting the use of these very widely used ASN migration features.

The migration features discussed here are useful to ISPs and
organizations of all sizes, but it is important to understand the
business need for these features and illustrate why they are critical, particularly so
critical for ISPs' operations. However, these features are not limited to ISPs and
organizations of all sizes use these features for similar reasons to
ISPs. During a merger, acquisition or
divestiture involving two organizations it is necessary to seamlessly
migrate both internal and external BGP speakers from one ASN to a
second ASN. The overall goal in doing so, particularly
in the case of a merger or acquisition, so is to achieve a uniform
operational model simplify operations
through consistent configurations across all BGP speakers in the
combined network. In addition, and perhaps more
imporantly, it is common practice in the industry
for ISPs to bill customers based on utilization. ISPs bill customers
based on the 95th percentile of the greater of the traffic sent or
received, over the course of a 1-month period, on the customer's PE-CE
access circuit. Given that the BGP Path Selection algorithm selects
routes with the shortest AS_PATH attribute, it is critical for that the
ISP to does not increase AS_PATH length during or after ASN migration, migration
toward
both downstream transit customers as well as or settlement-free peers, who are
likely sending or receiving traffic from those transit customers.
This would not only result in sudden changes in traffic patterns in
the network, but also (substantially) substantially decrease utilization driven
revenue at the ISP.

By default, the BGP protocol requires an operator to configure a
router to use a single remote ASN for the eBGP neighbor inside a router, BGP neighbor, and the ASN
must match on both ends of the peering in order to successfully
negotiate and establish an eBGP a BGP session. Prior to the existence of
these migration features, it would have required an ISP to work coordinate
an ASN change with, in some cases, tens of thousands of customers.
In particular, as each router is migrated to the new ASN, to avoid an
outage due to ASN mismatch, the ISP would have to encourage those force all customers
on that router to change their CE router configs configurations to use the new
ASN in a very short period of time,
when immediately after the customer has no business incentive to do so. ASN change. Thus, it becomes critical to
allow the ISP to make this process a bit more asymmetric, so that it
could seamlessly migrate the ASN within its network(s), but not disturb existing customers, and allow the
customers to gradually migrate to the ISP's new ASN at their leisure. leisure,
either by coordinating individual reconfigurations, or accepting
sessions using either the old or new ASN to allow for truly
asymmetric migration.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Documentation note

This draft uses Autonomous System Numbers (ASNs) from the range
reserved for documentation as described in RFC 5398 [RFC5398]. In
the examples used here, they are intended to represent Globally
Unique ASNs, not private use ASNs as documented in RFC 6996 [RFC6996]
section 10. 5.

2. ASN Migration Scenario Overview

The use case being discussed here is an ISP merging two or more ASNs,
where eventually one ASN subsumes the other(s). In this use case, we
will assume the most common case where there are two ISPs, A and B,
that prior to the ASN migration use AS 64500 and 64510, respectively, before the ASN migration
is to occur. respectively.
AS 64500 will be the permanently retained ASN used
going forward across the
consolidated set of both ISPs network
equipment equipment, and AS 64510 will be
retired. Thus, at the conclusion of the ASN migration, there will be
a single ISP A' with all internal BGP speakers configured to use AS
64500. To all external BGP speakers, the AS_PATH length will not be
increased.

In this same scenario, AS 64496 and AS 64499 represent two, two separate
customer networks: C and D, respectively. Originally, customer C (AS
64496) is attached to ISP B, which will undergo ASN migration from AS
64510 to AS 64500. Furthermore, customer D (AS 64499) is attached to
ISP A, which does not undergo ASN migration since ISP A's the ASN for ISP A
will remain constant, (AS 64500). Although this example refers to AS
64496 and 64499 as customer networks, either or both may be
settlement-free or other types of peers. In this use case they are
referred to as "customers" merely for convenience.

------ ------
/ ISP A \ / ISP B \
| AS 64500 | | AS 64510 |
\ / \ /
------- -------
| |
| |
------------ -------------
| Cust D | | Cust C |
| AS 64499 | | AS 64496 |
------------ -------------

Figure 1: Before Migration

---------------
/ \
| ISP A' |
| AS 64500 |
\ /
---------------
/ \
/ \
| |
------------ -------------
| Cust D | | Cust C |
| AS 64499 | | AS 64496 |
------------ -------------

Figure 2: After Migration

The general order of operations, typically carried out in a single
maintenance window by the network undergoing ASN migration, ISP B, migration (ISP B),
are as follows. First, ISP B, B will change the global BGP ASN used by
a PE Provider Edge (PE) router, from ASN 64510 to 64500. At this point,
the router will no longer be able to establish eBGP sessions toward
the existing CE Customer Edge (CE) devices that are attached to it and
still using AS 64510. Second, since ISP B will configure two separate, but related ASN migration features
discussed needs to do this without
coordinating the simultaneous change of its ASN with all of its eBGP
peers, ISP B will configure two separate, but related ASN migration
features discussed in this document on all eBGP sessions toward all
CE devices. These features enable the router to establish BGP
neighbors using the legacy ASN, modify the AS_PATH attribute received
from a CE device when advertising it further, and modify AS_PATH when
transmitted toward CE devices to achieve the desired effect of not
increasing the length of the AS_PATH.

At the conclusion of the ASN migration, the CE devices at the edge of
the network are not aware of the fact that their upstream router is
now in a new ASN and do not observe any change in the length of the
AS_PATH attribute. However, after the changes discussed in this
document are put in place by ISP A', there is a change to the
contents of the AS_PATH attribute to ensure the AS_PATH is not artifically
artificially lengthened for the duration of time that while these AS migration parameters are used.

In this use case, neither ISP is using BGP Confederations RFC 5065
[RFC5065] internally.

There are multiple implementations with equivalent features deployed
and in use. Some documentation pointers to these implementations, as
well as additional documentation on migration scenarios can be found
in the appendix. The examples cited below use Cisco IOS CLI for ease
of illustration purposes only.

3. External BGP Autonomous System Migration Features

The following section addresses features that are specific to
modifying the AS_PATH attribute at the Autonomous System Border
Routers (ASBRs) of an organization, (typically a single Service
Provider). This ensures that external BGP customers/peers are not
forced to make any configuration changes on their CE routers before
or during the exact time the Service Provider wishes to migrate to a
new, permanently retained ASN. Furthermore, these features eliminate
the artificial lengthening of the AS_PATH both transmitted from and
received by the Service Provider that is undergoing AS Migration,
which would have negative implications on path selection by external
networks.

3.1. Modify Inbound BGP AS_PATH Attribute

ISP B needs to reconfigure its router(s) to participate as an
internal BGP speaker in AS 64500, to realize the business goal of
becoming a single Service Provider: ISP A'. ISP B needs to do this
without coordinating the change of its ASN with all of its eBGP
peers, simultaneously.

The first step is for ISP B to change the
global AS in its router configuration, feature used by in the local BGP process
as the system-wide Autonomous System ID, from AS 64510 to AS 64500.
The next step is for ISP B to establish iBGP sessions with ISP A's
existing routers, thus consolidating ISP B into ISP A resulting in
operating under a single AS: ISP A', (AS 64500).

The next step described above is for ISP B to reconfigure its PE router(s) so that
each of its eBGP sessions toward all eBGP speakers with a feature called
"Local AS". This feature allows ISP B's the PE router that was formerly in
ISP B to re-
establish a re-establish an eBGP session toward the existing CE devices
using the legacy AS, AS 64510, in the eBGP session establishment. 64510. Ultimately, the CE devices, devices (i.e.:
customer C), C) are completely unaware that ISP B has reconfigured its
router to participate as a member of a new AS. Within the context of
the former ISP B's B PE router, The the second effect this feature has on
AS_PATH is that, by default, it prepends all received BGP UPDATEs
with the legacy AS of ISP B: AS 64510, while advertising
(Adj-RIB-Out) it (Adj-RIB-
Out) to other BGP speakers (A'). Thus, within Within Loc-RIB on ISP B' B prior to
the migration, the AS_PATH toward customer C would appear as: 64510,
whereas the same RIB on ISP A' (ISP B routers post-migration) would
contain AS_PATH: 64510 64496, which is
an increase in 64496. To avoid changes to the AS_PATH length from previously. Therefore,
length, a secondary feature "No Prepend" is required to be added to the "Local AS"
configuration toward every eBGP neighbor on ISP B's PE router. routers migrating from
ISP B. The "No Prepend" feature causes ISP B's PE router those routers to not prepend
the legacy AS, AS 64510, when advertising UPDATES received from
customer C. This restores the AS_PATH within ISP A' toward customer
C so that it is just one ASN in length: 64496.

In the direction of CE -> PE (inbound):

1. 'local-as <old_ASN>': prepends the <old_ASN> value to the AS_PATH
when advertising routes received from the CE

2. 'local-as <old_ASN> no-prepend': does not prepend <old_ASN> value
to the AS_PATH when advertising routes received from the CE

As stated previously, local-as <old_ASN> no-prepend, (configuration
#2), is critical because it does not increase the AS_PATH length.
Ultimately, this ensures that routes learned from ISP B's legacy
customers will be transmitted through legacy eBGP sessions of ISP A,
toward both customers and peers, will contain only two AS'es in the
AS_PATH: 64500 64496. Thus, the legacy customers and peers of ISP A
will not see an increase in (inbound):

1. 'local-as <old_ASN>': prepends the AS_PATH length <old_ASN> value to reach ISP B's
legacy customers. Ultimately, it is considered mandatory by
operators that both the "Local AS" and "No Prepend" configuration
parameters always be used in conjunction with each other in order AS_PATH
when advertising routes received from the CE

2. 'local-as <old_ASN> no-prepend': does not prepend <old_ASN> value
to
ensure the AS_PATH length is not increased.

PE-1 when advertising routes received from the CE

PE-B is a PE that was originally in ISP B. PE-1 B, and has a customer peer
CE-B. PE-B has had its global configuration ASN changed from AS
64510 to AS 64500 to make it part of the permanently retained ASN.
This now makes PE-1 PE-B a member of ISP A'. PE-2 PE-A is a PE that was
originally in ISP A. A, and has a customer peer CE-A. Although its
global configuration ASN remains AS 64500, throughout this exercise
we also consider PE-2 PE-A a member of ISP A'.

ISP A' ISP A'
CE-1 ---> PE-1 -------------------> PE-2 ---> CE-2
64496 Old_ASN: 64510
CE-A <--- PE-A <------------------- PE-B <--- CE-B
64499 New_ASN: 64500 64499 Old_ASN: 64510 64496
New_ASN: 64500

Note: Direction of BGP UPDATE as per the arrows.

Figure 3: Local AS BGP UPDATE Diagram

The final configuration on PE-1 PE-B after completing the "Local AS"
portion of the AS migration is as follows:

router bgp 64500
neighbor <CE-1_IP> <CE-B_IP> remote-as 64496
neighbor <CE-1_IP> <CE-B_IP> local-as 64510 no-prepend

As a result of the "Local AS No Prepend" configuration, on PE-1, CE-2 PE-B, CE-A
will see an AS_PATH of: 64500 64496. CE-2 CE-A will not receive a BGP
UPDATE containing AS 64510 in the AS_PATH. (If only the "local-as
64510" feature was configured without the keyword "no-prepend" on PE-
1,
B, then CE-2 CE-A would see an AS_PATH of: 64496 64510 64500, which
results in an unacceptable lengthening of the AS_PATH).

3.2. Modify Outbound BGP AS_PATH Attribute

The previous feature, "Local AS No Prepend", was only designed to modify
the AS_PATH Attribute received by the ISP in updates from CE devices,
when CE devices still have an eBGP session established with the ISPs
legacy AS, (AS64510). In some existing implementations, "Local AS No
Prepend" does not concurrently modify the AS_PATH Attribute for BGP
UPDATEs that are transmitted by the ISP to CE devices. Specifically,
with "Local AS No Prepend" enabled on ISP A's
PE-1, PE-B, it automatically causes a
lengthening of the AS_PATH in outbound BGP UPDATEs from ISP A' toward
directly attached eBGP speakers, (Customer C in AS 64496). This is
the result of the "Local AS No Prepend" feature automatically
appending the new global configuration ASN, AS64500, after the legacy
ASN, AS64510, on ISP A'
PE-1 in BGP UPDATEs that are transmitted by PE-1 PE-B to CE-1. CE-B.
The end result is that customer C, in AS 64496, will receive the
following AS_PATH: 64510 64500 64499. Therefore, if ISP A' takes no
further action, it will cause an unacceptable increase in AS_PATH
length within customer's networks directly attached to ISP A'.

A second feature was designed to resolve this problem (continuing the
use of Cisco CLI in the examples, it is called "Replace AS" in the
examples below). This feature allows ISP A' to prevent routers
configured with this feature from appending the global configured AS
in outbound BGP UPDATEs toward its customer's networks configured
with the "Local AS" feature. Instead, only the historical (or
legacy) AS will be prepended in the outbound BGP UPDATE toward the
customer's network, restoring the AS_PATH length to what it what was
before AS Migration occurred.

To re-use the above diagram, but in the opposite direction, we have:

ISP A' ISP A'
CE-1 <--- PE-1 <------------------- PE-2 <--- CE-2
64496 Old_ASN: 64510
CE-A ---> PE-A -------------------> PE-B ---> CE-B
64499 New_ASN: 64500 64499 Old_ASN: 64510 64496
New_ASN: 64500

Note: Direction of BGP UPDATE as per the arrows.

Figure 4: Replace AS BGP UPDATE Diagram

The final configuration on PE-1 PE-B after completing the "Replace AS"
portion of the AS migration is as follows:

router bgp 64500
neighbor <CE-1_IP> <CE-B_IP> remote-as 64496
neighbor <CE-1_IP> <CE-B_IP> local-as 64510 no-prepend replace-as

By default, without "Replace AS" enabled, CE-1 CE-B would see an AS_PATH
of: 64510 64500 64499, which is artificially lengthened by the ASN
Migration. After ISP A' changes PE-1 PE-B to include the "Replace AS"
feature, CE-1 CE-B would receive an AS_PATH of: 64510 64499, which is the
same AS_PATH length pre-AS migration.

3.3. Implementation

While multiple implementations already exist, the following should
document the expected behavior such that a new implementation of this
feature could be done on other platforms.

These features MUST be configurable on a per-neighbor or per peer-
group basis to allow for maximum flexibility. When this feature set
is invoked, an ASN that is different from the globally-configured ASN
is provided as a part of the command as exemplified above. To
implement this feature, a BGP speaker MUST send BGP OPEN messages to
the configured eBGP peer using the ASN configured for this session as
the value sent in MY ASN. The speaker MUST NOT use the ASN
configured globally within the BGP process as the value sent in MY
ASN in the OPEN message. This will avoid the BGP OPEN Error message
BAD PEER AS, and is typically used to re-establish eBGP sessions with
peers expecting the legacy ASN after a router has been moved to a new
ASN. Additionally, when the BGP speaker configured with this feature
receives updates from its neighbor, it MUST process the update as
normal, but it MUST append the configured ASN in the AS_PATH
attribute before advertising the UPDATE to any other BGP speaker.
Note that processing the update as normal will include appending the
globally configured ASN to the AS_PATH, thus processing this update
will result in the addition of two ASNs to the AS_PATH attribute.
Similarly, for outbound updates sent by the configured BGP speaker to
its neighbor, the speaker MUST append the configured ASN to the
AS_PATH attribute, adding to the existing global ASN in the AS_PATH,
for a total of two ASNs added to the AS_PATH.

Two options exist to manipulate the behavior of this feature. They
modify the behavior as described below:

No prepend inbound - When the BGP speaker configured with this option
receives inbound updates from its neighbor, it MUST NOT append the
configured ASN in the AS_PATH attribute when advertising that UPDATE
to other peers and instead MUST append only the globally configured
ASN.

No prepend outbound - When the BGP speaker configured with this
option generates outbound BGP updates to the configured peer, the BGP
speaker MUST remove the globally configured ASN from the AS_PATH
attribute, and MUST append the locally configured ASN to the AS_PATH
attribute before sending outbound BGP updates to the configured peer.

While the exact command syntax is an implementation detail beyond the
scope of this document, the following consideration may be helpful
for implementers: Implementations MAY integrate the behavior of the
options described above into a single command that addresses both
inbound and outbound updates, but if this is done, implementations
MUST provide a method to select its applicability to inbound updates,
outbound updates, or updates in both directions. Several existing
implementations use separate commands (e.g. local-as no-prepend vs
local-as replace-as) for maximum flexibility in controlling the
behavior on the session to address the widest range of possible
migration scenarios.

4. Internal BGP Autonomous System Migration Features

The following section describes features that are specific to
performing an ASN migration within medium to large networks in order
to realize the business and operational benefits of a single network
using one, globally unique Autonomous System. These features assist with a gradual
and least service impacting migration of Internal BGP sessions from a
legacy ASN to the permanently retained ASN. It
should be noted that the The following feature is
very valuable to networks undergoing AS migration, but its use does
not cause changes to the AS_PATH attribute.

4.1. Internal BGP Alias

In this case, all of the routers to be consolidated into a single,
permanently retained ASN are under the administrative control of a
single entity. Unfortunately, the traditional method of migrating
all Internal BGP speakers, particularly within larger networks, is
both time consuming and widely service impacting.

The traditional method to migrate Internal BGP sessions was strictly
limited to reconfiguration of the global configuration ASN and,
concurrently, changing of all iBGP neighbor's neighbors' remote ASN from the legacy
ASN to the new, permanently retained ASN on each router within the
legacy AS. These changes can be challenging to swiftly execute in
networks with with more than a few dozen internal BGP speakers.
There is also the concomitant service interruptions as these changes
are made to routers within the network, resulting in a reset of iBGP
sessions and subsequent route reconvergence times to reestablish optimal
routing paths. Operators do not, and in some cases, often cannot make such sweeping changes
given the associated risks and of a highly visible service interruption;
rather, they require a more gradual method to migrate Internal BGP
sessions, from one ASN to a second, permanently retained ASN, that is
not visibly service-impacting to its customers.

With the "Internal BGP Alias" [JUNIPER] feature, it allows an
Internal BGP speaker to form a single iBGP session using either the
old, legacy ASN or the new, permanently retained ASN. The benefits
of using this feature are several fold. First, it allows for a more
gradual and less service-impacting migration away from the legacy ASN
to the permanently retained ASN. Second, it (temporarily) permits
the coexistence of the legacy and permanently retained ASN within a
single network, allowing for uniform BGP path selection among all
routers within the consolidated network. NB: Cisco doesn't have an
exact equivalent to "Internal BGP Alias", but the combination of the
Cisco features iBGP local-AS and dual-as provides similar
functionality.

When the "Internal BGP Alias" feature is enabled, typically just on
one side of a iBGP session, it allows that iBGP speaker to establish
a single iBGP session with either the legacy ASN or the new,
permanently retained ASN, depending on which one it receives in the
"My Autonomous System" field of the BGP OPEN message from its iBGP
session neighbor. It is important to recognize that enablement of
the "Internal BGP Alias" feature preserves the semantics of a regular
iBGP session, (using identical ASNs). Thus, the BGP attributes
transmitted by and the acceptable methods of operation on BGP
attributes received from iBGP sessions configured with "Internal BGP
Alias" are no different than those exchanged across an iBGP session
without "Internal BGP Alias" configured, as defined by [RFC4271] and
[RFC4456].

Typically, in medium to large networks, BGP Route Reflectors
[RFC4456] (RRs) are used to aid in reduction of configuration of iBGP
sessions and scalability with respect to overall TCP (and, BGP)
session maintenance between adjacent iBGP speakers. Furthermore, BGP
Route Reflectors are typically deployed in pairs within a single
Route Reflection cluster to ensure high reliability of the BGP
Control Plane. As such, the following example will use Route
Reflectors to aid in understanding the use of the "Internal BGP
Alias" feature. Note that Route Reflectors are not a prerequisite to
enable "Internal BGP Alias" and this feature can be enabled
independent of the use of Route Reflectors.

The general order of operations is as follows:

1. Within the legacy network, (the routers comprising the set of
devices that still have a globally configured legacy ASN), take one
member of a redundant pair of RRs and change has its global configuration
ASN changed to the permanently retained ASN. Concurrently,
enable use of
"Internal BGP Alias" is configured on all iBGP sessions. This
will comprise Non-Client iBGP sessions to other RRs as well as
Client iBGP sessions, typically to PE devices, both still
utilizing the legacy ASN. Note that during this step there will
be a reset and reconvergence event on all iBGP sessions on the
RRs whose configuration was modified; however, this should not be
service impacting due to the use of redundant RRs in each RR
Cluster.

2. Repeat the The above step is repeated for the other side of the redundant
pair of RRs. The one alteration to the above procedure is to disable use
of that
"Internal BGP Alias" on is now removed from the Non-Client iBGP
sessions toward the other (previously reconfigured) RRs, since it
is no longer needed. "Internal BGP Alias" is still required on
all RRs for all RR Client iBGP sessions. Also during this step,
there will be a reset and reconvergence event on all iBGP
sessions whose configuration was modified, but this should not be
service impacting. At the conclusion of this step, all RRs
should now have their globally configured ASN set to the
permanently retained ASN and "Internal BGP Alias" enabled and in
use toward RR Clients.

3. At this point, the network administrators would then be able to
establish iBGP sessions between all Route Reflectors in both the
legacy and permanently retained networks. This would allow the
network to appear to function, both internally and externally, as
a single, consolidated network using the permanently retained
network.

4. The next steps to To complete the AS migration are to gradually
modify migration, each RR Client, (PE), Client (PE) in the legacy
network still utilizing the legacy ASN. ASN is now modified.
Specifically, each legacy PE would have its globally configured
ASN changed to use the permanently retained ASN. The ASN used by
the PE for the iBGP sessions, sessions toward each RR, RR would be changed to
use the permanently retained ASN. (It is unnecessary to enable
"Internal BGP Alias" on the migrated iBGP sessions). During the
same maintenance window, External BGP sessions would be modified
to include the above "Local AS No Prepend" and "Replace-AS" features,
features described in Section 3 above, since all of the changes
are service interrupting to the eBGP sessions of the PE. At this
point, all PE's PEs will have been migrated to the permanently
retained ASN.

5. The final step is to excise the "Internal BGP Alias"
configuration from the first half of the legacy RR Client pair --
this will expunge "Internal BGP Alias" configuration from all
devices in the network. After this is complete, all routers in
the network will be using the new, permanently retained ASN for
all iBGP sessions with no vestiges of the legacy ASN on any iBGP
sessions.

The benefit of using "Internal BGP Alias" is that it is a more
gradual and less externally visible, service-impacting change to accomplish an
AS migration. Previously, without "Internal BGP Alias", such an AS
migration change would carry a high risk and need to be successfully
accomplished in a very short timeframe, timeframe (e.g.: at most several hours).
In addition, it would likely cause substantial routing churn and, likely, and
rapid fluctuations in traffic carried -- potentially causing periods
of congestion and resultant packet loss -- during the period the
configuration changes are underway to complete the AS Migration. On
the other hand, with "Internal BGP Alias", the migration from the
legacy ASN to the permanently retained ASN can occur over a period of
days or weeks with little disruption
experienced by customers of the network undergoing AS migration. reduced customer disruption. (The only observable
service disruption should be when each PE undergoes the changes
discussed in step 4 above.)

4.2. Implementation

When configured with this feature, a BGP speaker MUST accept BGP OPEN
and establish an iBGP session from configured iBGP peers if the ASN
value in MY ASN is either the globally configured ASN or the locally
configured ASN provided in this command. Additionally, a BGP speaker
configured with this feature MUST send its own BGP OPEN using both
the globally configured and the locally configured ASN in MY ASN. To
avoid potential deadlocks when two BGP speakers are attempting to
establish a BGP peering session and are both configured with this
feature, the speaker SHOULD send BGP OPEN using the globally
configured ASN first, and only send a BGP OPEN using the locally
configured ASN as a fallback if the remote neighbor responds with the
BGP error BAD PEER ASN. In each case, the BGP speaker MUST treat
updates sent and received to this peer as if this was a natively
configured iBGP session, as defined by [RFC4271] and [RFC4456].

Implementations of this feature MAY integrate the functionality from
the eBGP features (Section 3) section as a part of this command in
order to simplify support for eBGP migrations as well as iBGP
migrations, such that an eBGP session to a configured neighbor could
be established via either the global ASN or the locally configured
ASN. If the eBGP session is established with the global ASN, no
modifications to AS_PATH are required, but if the eBGP session is
established with the locally configured ASN, the modifications
discussed in eBGP features (Section 3) MUST be implemented to
properly manipulate the AS_PATH.

5. Additional Operational Considerations

This document describes several features to support ISPs and other
organizations that need to perform ASN migrations. Other variations
of these features may exist, for example, in legacy router software
that has not been upgraded or reached End of Life, but continues to
operate in the network. Such variations are beyond the scope of this
document.

Companies routinely go through periods of mergers, acquisitions and
divestitures, which in the case of the former cause them to
accumulate several legacy ASNs over time. ISPs often do not have
control over the configuration of customer's devices, customers' devices (i.e.: the ISPs
are often not providing a managed CE router service, particularly to
medium and large customers that require eBGP). Furthermore, ISPs are
using methods to perform ASN migration that do not require
coordination with customers. Ultimately, this means there is not a
finite period of time after which legacy ASNs will be completely
expunged from the ISP's network. In fact, it is common that legacy
ASNs and the associated External BGP AS Migration features discussed
in this document can and do persist for several years, if not longer.
Thus, it is prudent to plan that legacy ASNs and associated External
BGP AS Migration features will persist in a operational network
indefinitely.

With respect to the Internal BGP AS Migration Features, all of the
routers to be consolidated into a single, permanently retained ASN
are under the administrative control of a single entity. Thus,
completing the migration from iBGP sessions using the legacy ASN to
the permanently retained ASN is more straightforward and could be
accomplished in a matter of days to months. Finally, good
operational hygiene would dictate that it is good practice to avoid
using "Internal BGP Alias" over a long period of time for reasons of
not only operational simplicity of the network, but also reduced
reliance on that feature during the ongoing lifecycle management of
software, features and configurations that are maintained on the
network.

6. Conclusion

Although the features discussed in this document are not formally
recognized as part of the BGP4 specification, they have been in
existence in commercial implementations for well over a decade.
These features are widely known by the operational community and will
continue to be a critical necessity in the support of network
integration activities going forward. Therefore, these features are
extremely unlikely to be deprecated by vendors. As a result, these
features must be acknowledged by protocol designers, particularly
when there are proposals to modify BGP's behavior with respect to
handling or manipulation of the AS_PATH Attribute. More
specifically, assumptions should not be made with respect to the
preservation or consistency of the AS_PATH Attribute as it is
transmitted along a sequence of ASN's. ASNs. In addition, proposals to
manipulate the AS_PATH that would gratuitously increase AS_PATH
length or remove the capability to use these features described in
this document will not be accepted by the operational community.

7. Acknowledgements

Thanks to Kotikalapudi Sriram, Stephane Litkowski, Terry Manderson,
David Farmer, Jaroslaw Adam Gralak, Gunter Van de Velde, and Juan
Alcaide
Alcaide, Jon Mitchell, and Thomas Morin for their comments.

8. IANA Considerations

This memo includes no request to IANA.

9. Security Considerations

This draft discusses a process by which one ASN is migrated into and
subsumed by another. This involves manipulating the AS_PATH
Attribute with the intent of not increasing the AS_PATH length, which
would typically cause the BGP route to no longer be selected by BGP's
Path Selection Algorithm in other's others' networks. This could result in a
loss of revenue if the ISP is billing based on measured utilization
of traffic sent to/from entities attached to its network. This could
also result in sudden and unexpected shifts in traffic patterns in
the network, potentially resulting in congestion, in the most extreme
cases.

Given that these features can only be enabled through configuration
of router's routers within a single network, standard security measures should
be taken to restrict access to the management interface(s) of routers
that implement these features.

10. Appendix: Implementation report

As noted elsewhere in this document, this set of migration features
has multiple existing implementations in wide use.

o Cisco [CISCO]

o Juniper [JUNIPER]

o Alcatel-Lucent [ALU]

This is not intended to be an exhaustive list, as equivalent features
do exist in other implementations, however the authors were unable to
find publicly available documentation of the vendor-specific
implementation to reference.

11. References

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.

[RFC5398] Huston, G., "Autonomous System (AS) Number Reservation for
Documentation Use", RFC 5398, December 2008.

11.2. Informative References

[ALU] Alcatel-Lucent, "BGP Local AS attribute", 2006-2012,
<https://infoproducts.alcatel-lucent.com/html/0_add-
h-f/93-0074-10-01/7750_SR_OS_Routing_Protocols_Guide/BGP-
CLI.html#709567>.

[CISCO] Cisco Systems, Inc., "BGP Support for Dual AS
Configuration for Network AS Migrations", 2003,
<http://www.cisco.com/c/en/us/td/docs/ios-
xml/ios/iproute_bgp/configuration/xe-3s/asr1000/
irg-xe-3s-asr1000-book/irg-dual-as.html>.

[JUNIPER] Juniper Networks, Inc., "Configuring the BGP Local
Autonomous System Attribute", 2012,
<http://www.juniper.net/techpubs/en_US/junos13.3/topics/
concept/bgp-local-as-introduction.html>.

[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.

[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007.

[RFC6996] Mitchell, J., "Autonomous System (AS) Reservation for
Private Use", BCP 6, RFC 6996, July 2013.

Authors' Addresses

Wesley George
Time Warner Cable
13820 Sunrise Valley Drive
Herndon, VA 20171
US

Phone: +1 703-561-2540
Email: wesley.george@twcable.com

Shane Amante
Apple, Inc.
1 Infinite Loop
Cupertino, CA 95014
US

Email: samante@apple.com