wdiff draft-ietf-idr-best-external-03.txt draft-ietf-idr-best-external-04.txt

Network Working Group P. Marques
Internet-Draft
Intended status: Standards Track R. Fernando
Expires: September 10, October 22, 2011 R. Fernando
E. Chen
P. Mohapatra
Cisco Systems
March 9,
H. Gredler
Juniper Networks
April 20, 2011

Advertisement of the best external route in BGP
draft-ietf-idr-best-external-03.txt
draft-ietf-idr-best-external-04

Abstract

The base BGP specifications prevent a BGP speaker from advertising
any route current BGP-4 protocol specification [RFC4271] states that is not the
selection process chooses the best route path for a BGP destination. This
document specifies a modification of this rule. Routes are divided
into two categories, "external" and "internal". A specification given route which is
provided for choosing a "best external route" (for a particular value
added to the Loc-Rib and advertised to all peers.

Previous versions [RFC1771] of the Network Layer Reachability Information). A specification defined a different
rule for Internal BGP speaker is
then allowed to advertise its "best external route" Updates. Given that Internal paths are not re-
advertised to its internal
BGP Internal peers, even if it was specified that is not the best route for of the destination.
The
external paths, as determined by the path selection tie breaking
algorithm, would be advertised to Internal peers.

This document extends that procedure to operate in environments where
Route Reflection [RFC4456] or Confederations [RFC5065] are used and
explains why advertising the best external route additional routing information can
improve convergence time without causing routing loops.

Additional benefits include reduction of inter-domain churn and
avoidance of permanent route oscillation. The document also generalizes the
notions of "internal" and "external" so that they can be applied to
Route Reflector Clusters and Autonomous System Confederations.

Status of this Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
2.
3. Generalization . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Algorithm for selection of best external route . the Adj-RIB-OUT path . . . . . . . 5
3. 7
5. Advertisement Rules . . . . . . . . . . . . . . . . . . . . . 6
4. 9
6. Consistency between routing and forwarding . . . . . . . . . . 6
5. 10
7. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. 12
8. Fast Connectivity Restoration . . . . . . . . . . . . . . 8
5.2. . . 13
9. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . 9
5.3. . . 14
10. Reducing Persistent IBGP oscillation . . . . . . . . . . . 9
6. . . 15
11. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
7. 16
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. 18
14. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Normative 19
15. References . . . . . . . . . . . . . . . . . . . . . 10 . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 21

1. Introduction

The base BGP specifications prevent a BGP speaker

Earlier versions of the BGP-4 protocol specification [RFC1771]
prescribed different route advertisement rules for Internal and
External peers. While the overall best path would be advertised to
External peers, Internal peers are advertised the best of the
externally received paths.

This Internal advertisement rule was never implemented as specified
and was latter dropped from the protocol. There is a trade-off in
advertising
any the "best-external" route versus the behavior that is became
common standard of not advertising the best route for a BGP destination. This
document specifies a modification of when the selected best
path is received from an Internal peer. By not advertising
information in this rule. Routes are divided
into two categories, "external" and "internal". A specification case it is
provided for choosing a "best external route" (for a particular value
of possible to reduce state both in the Network Layer Reachability Information). A
local BGP speaker is
then allowed to advertise its "best external route" to its internal as well as in the network overall. Early BGP peers, even if that
implementations where very concerned with reducing state as they
where limited to relatively low memory footprints (e.g. 16 MB).
There is not also the best route for possible concern regarding advertising a path
different than the destination.
The document explains why path that has been selected for forwarding.

However, advertising the best external route can
improve convergence time without causing routing loops. Additional
benefits include reduction of inter-domain churn and avoidance of
permanent route oscillation.

The document also generalizes route, when different from the notions
best route, presents additional information into an IBGP mesh which
may be of "internal" and
"external" so value for several purposes including:

o Faster restoration of connectivity. By providing additional
paths, that they can may be applied used to Route Reflector Clusters
[RFC4456] and Autonomous System Confederations [RFC5065]. More
specifically, two routers fail over in the same route reflector cluster having
an IBGP session between them are defined to be "internal" peers,
whereas two routers in different clusters having an IBGP session are
defined to be "external" peers. Similarly, two routers in the same
member AS of a confederation having an IBGP session between them are
"internal" peers, whereas two routers in different member ASs of a
confederation having a confed EBGP session between them are defined
to be "external" peers. The definition of "best external route"
ensues from this definition in that it is the most preferred route
among those received from the "external" neighbors.

Advertising the best external route, when different from the best
route, presents additional information into an IBGP mesh which may be
of value for several purposes including:

o Faster restoration of connectivity, by providing additional paths,
that may be used to fail over in case case the primary path
becomes invalid or is withdrawn.

o Reducing inter-domain churn and traffic blackholing black-holing due to the
readily available alternate path.

o Reducing the potential for situations of permanent IBGP route
oscillation, as discussed in some scenarios
oscillation [RFC3345].

o Improving selection of lower MED routes from the same neighboring
AS.

This document defines procedures to select the best external route
for each destination. peer. It also describes how above benefits are realized
with best external route announcement with the help of certain
scenarios.

1.1.

2. 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].

2. Algorithm for selection of best external route

Given

3. Generalization

The BGP-4 protocol [RFC1771] has extended with two alternative
mechanisms that provide ways to reduce the intent in advertising operational complexity of
route distribution within an external route, when the best AS: Route Reflection [RFC4456] and
Confederations [RFC5065]. It is important to be able to express
route for advertisement rules in the same destination context of both of these mechanisms.

When Route Reflection is an used, Internal peers are further classified
depending of the reflection cluster they belong to. Non-client
internal route, is to provide
additional information into peers form one BGP peering mesh. Each set of RR clients
with the IBGP mesh into which same "cluster-id" configuration forms a route is
participating, it is desirable separate mesh.

When selecting the path to add to take into account the routes
received Adj-RIB-OUT, this document
specifies that the path that originate from internal neighbors in the same mesh MAY be
excluded from consideration. This results in an Adj-RIB-OUT
selection process.

We propose per mesh (the set of non-client peers or a route selection algorithm that selects specific
cluster).

Similarly, when BGP Confederations are used, each confederated AS is
a total order
between BGP mesh. As with the Route Reflection scenario, when selecting
the path to add to the Adj-RIB-OUT, routes and which selects from the same mesh MAY be
excluded.

4. Algorithm for selection of the Adj-RIB-OUT path

The objective of this protocol extension is to improve the quality of
the routing information known to a particular BGP mesh with minimum
additional cost in terms of processing and state.

Towards that goal, it is useful to define a total order between the
Adj-RIB-In routes which provides both the same overall best route path as
the one
currently specified [RFC4271]. algorithm defined in the current BGP-4 specification [RFC4271] as
well as an ordering of alternate routes. Using this total order it
is then computationally efficient to select the path for a specific
Adj-RIB-OUT by excluding the routes that have been received from the
BGP mesh corresponding to the peer (or set of peers).

In order to achieve this, we need it is helpful to introduce the concept of route
path group. For a given NLRI, suppose A group is the BGP decision process has run set of paths that compare as equal
through all the steps prior to the MED comparison step (as defined in
section 9.1.2.2 of [RFC4271]. Look at the set of routes that are
still under consideration at that time. Now partition this set into
a number of disjoint route groups, where two routes are in the same
group if RFC 4271 [RFC4271] and only if have been received from the
same neighbor AS of each route is the same.

Routes AS.

Paths are ordered within a group via MED or subsequent route
selection rules.

The order of all routes for the same destination is determined by the
order of

In pseudo-code:

function compare(path_1, path_2) {
cmp_result cmp = selection_steps_before_med(path_1, path_2);
if (cmp != cmp_result.equal) {
return cmp;
}
if (neighbor_as(path_1) == neighbor_as(path_2)) {
return selection_steps_after_med(path_1, path_2);
}

if (is_group_best(path_1)) {
if (!is_group_best(path_2)) {
return cmp_result.greater_than;
}
return selection_steps_after_med(path_1, path_2);
} else {
if (is_group_best(path_2)) {
return cmp_result.less_than;
}
/* Compare the best route in each group. paths of respective groups */
return compare(group_best(path_1), group_best(path_2));
}
}

As an example, the following set of received routes:

+------+----+-----+--------+
| Path | AS | MED | rtr_id |
+------+----+-----+--------+
| a | 1 | 10 | 10 |
| | | | |
| b | 2 | 5 | 1 |
| | | | |
| c | 1 | 5 | 5 |
| | | | |
| d | 2 | 20 | 20 |
| | | | |
| e | 2 | 30 | 30 |
| | | | |
| f | 3 | 10 | 20
Figure 1: |
+------+----+-----+--------+

Path Attribute Table

Would yield the following order (from the most to the least
preferred):

b < d < e < c < a < f

In this example, comparison of the best route path within each group
provides the sequence (b < c < f). The remaining routes paths are ordered
in relation to their respective group best.

The first route path in the above ordering above is indeed the best route overall path for a
given destination. Eliminating the best route and executing the
above steps leads us to NLRI. When selecting a new total order of the routes. The route
to be advertised to path for a particular domain is selected by choosing the
most preferred route that is external Adj-RIB-Out (or
set of RIB-Outs) an implementation MAY choose to that particular domain select the first
path in the above order. Note that whenever global order which was not received from the overall best route is
external it will automatically be selected by this algorithm.

3. same BGP
mesh (as defined above) as the target peer (or peers).

5. Advertisement Rules

1. In an AS domain, if When advertising a router has installed an internal route as
best, it should advertise its "best external route" (as defined
in the draft) to its internal neighbors.

2. In a Cluster domain, if non-client Internal peer, a router (route reflector) has installed
an external route as best, it should advertise its "best internal
route" to its external neighbors. (Advertising BGP
speaker MAY choose to internal
neighbors is unchanged.) Similarly, if select the route reflector has
installed an internal route as best, it should advertise its
"best external route" to its internal (client) peers. In first path in order
for that did not
originate from the same BGP mesh (i.e. the set of non-client
Internal peers) whenever the best overall path has been received
from this mesh and would be suppressed by the reflector Internal BGP non-
readvertisement rule.

2. When advertising a route to a Route Reflection client peer, in
case the overall best path has been received from the same
cluster, a BGP speaker MUST be able to advertise the best external route
into overall
path to all the cluster, it members of the cluster other than the originator,
unless "client-to-client" reflection is necessary disabled. The
implementation MAY choose to advertise an alternate path to the
specific peer that client-to-client
reflection be disabled, since its advertisement may otherwise
contain originates the best route within overall path by excluding
from consideration all paths with the cluster domain. same originator-id.

3. In a Confederation Member domain, if a router (confederation
border router) has installed an internal route as best, it
advertises its best external route to its internal neighbors.
However, if it has installed an external route as best, it
advertises its best internal route to its external neighbors.

4. Consistency between routing When "client-to-client" reflection is disabled and forwarding

The BGP protocol, the cluster is
operating as defined in [RFC1771], specifies that a BGP
speaker shall mesh, a Route Reflector MAY opt to advertise to its internal peers
the route with cluster the
highest degree of preference among routes to preferred path from the same destination set of paths not received
from external neighbors.

This section discusses problems present with the approach described
in [RFC1771] and the next section offers an alternative algorithm to
select a best external route which cluster. While this deployment mode is currently
uncommon, it can be advertised a practical way to guarantee path diversity
inside the cluster.

4. A confederation border route MAY choose to advertise an IBGP mesh. alternate
path towards its Internal BGP mesh or towards a con-fed member AS
following the same procedure as defined above.

6. Consistency between routing and forwarding

The internal update advertisement rules contained in the original
BGP-4 specification [RFC1771] can lead to situations where traffic is
forwarded through a route other than the route advertised by BGP.

Inconsistencies between forwarding and routing are highly
undesirable. Service providers use BGP with the dual objective of
learning reachability reach-ability information and expressing policy over network
resources. The latter assumes that forwarding follows routing
information.

Consider the Autonomous system presented in figure 1, where r1 ... r4
are members of a single IBGP mesh and routes a, b, and c are received
from external peers.

AS 1 (c)
|
+----+ +----+
| r1 |...........| r2 |
+----+ +----+
.
.
.
.
.
.
+----+ +----+
| r3 |...........| r4 | --- ebgp --- AS X
+----+ +----+
/ \
/ \
AS 1 (a) AS 2 (b)

Figure 2:

Inconsistency in Routing

+------+----+-----+--------+
| Path | AS | MED rtr_id | rte_id |
+------+----+-----+--------+
| a | 1 | 10 | 1 |
| | | | |
| b | 2 | 5 | 10 |
| | | | |
| c | 1 | 5 | 5

Figure 3: |
+------+----+-----+--------+

Path Attribute Table - 2

Following the rules as specified in RFC 1771 [RFC1771], router r3
will select path (b) received from AS 2 as its overall best to
install in the Loc-Rib, since path (b) is preferable to path (c), the
lowest MED route from AS 1. However for the purposes of Internal
Update route selection, it will ignore the presence of path (c), and
elect (a) as its advertisement, via the router-id tie-breaking rule.

In this scenario, router r4 will receive (c) from r1 and (a) from r3.
It will pick the lowest MED route (c) and advertise it out via ebgp IBGP
to AS X. However at this point routing is inconsistent with
forwarding as traffic received from AS X will be forwarded towards AS
2, while the ebgp IBGP advertisement is being made for an AS 1 path.

Routing policies are typically specified in terms of neighboring
ASes.
AS-es. In the situation above, assuming that AS 1 is network for
which this AS provides transit services while AS 2 and AS X are peer
networks, one can easily see how the inconsistency between routing
and forwarding would lead to transit being inadvertently provided
between AS X and AS 2. This could lead to persistent forwarding
loops.

Inconsistency between routing and forwarding may happen, whenever a
bgp
GP speaker chooses to advertise an external route into IBGP that is
different from the overall best route and its overall best is
external.

7. Applications

5.1.
8. Fast Connectivity Restoration

When two exits are available to reach a particular destination and
one is preferred over the other, the availability of an alternate
path provides fast connectivity restoration when the primary path
fails.

Restoration can be quick since the alternate path is already at hand.
The border router could precompute recompute the backup route and preinstall perinatal it
in FIB ready to be switched when the primary goes away. Note that
this requires the border router that's the backup to also preinstall perinatal
the secondary path and switch to it on failure.

5.2.

9. Inter-Domain Churn Reduction

Within an AS, the non availability of backup best leads to a border
router sending a withdraw upstream when the primary fails. This
leads to inter-domain churn and packet loss for the time the network
takes to converge to the alternate path. Having the alternate path
will reduces the churn and eliminates packet loss.

5.3.

10. Reducing Persistent IBGP oscillation

Advertising the best external route, according to the algorithm
described in this document will reduce the possibility of route
oscillation by introducing additional information into the IBGP
system.

For a permanent oscillation condition to occur, it is necessary that
a circular dependency between paths occurs such that the selection of
a new best path by a router, in response to a received IBGP
advertisement, causes the withdrawal of information that another
router depends on in order to generate the original event.

In vanilla BGP, when only the best overall route is advertised, as in
most implementations, oscillation can occur whenever there are 2 or
clusters/sub-ASes
clusters/sub-AS-es such that at least one cluster has more than one
path that can potentially contribute to the dependency.

11. Deployment Considerations

The mechanism specified in the draft allows a BGP speaker to
advertise a route that is not the best route used for forwarding.
This is a departure from the current behavior. However, consistency
in the path selection process across the AS is still guaranteed since
the ingress routers will not choose the best-external route as the
best route for a destination in steady state (for the same reason
that the BGP speaker announcing the best-external route chose an IBGP
route as best instead of the externally learnt route). Though it is
possible to alter this assurance by defining route policies on IBGP
sessions, use of such policies in IBGP is not recommended, especially
with best-external announcement turned on in the network. It is also
worth noting that such inconsistency in routing and forwarding is
mitigated in a tunneled network.

12. Acknowledgments

This document greatly benefits from the comments of Yakov Rekhter,
John Scudder, Eric Rosen, Jenny Yuan, and Jay Borkenhagen.

8. Borkenhagen, Salkat Ray and
Jakob Heitz.

13. IANA Considerations

This document has no actions for IANA.

14. Security Considerations

There are no additional security risks introduced by this design.

10. Normative

15. References

[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.

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

[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, August 2002.

[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.

Authors' Addresses

Pedro Marques

Email: pedro.r.marques@gmail.com

Rex Fernando
Cisco Systems
170 W. Tasman Drive Dr.
San Jose, CA 95134
USA
US

Email: rex@cisco.com

Enke Chen
Cisco Systems
170 W. Tasman Drive Dr.
San Jose, CA 95134
USA
US

Email: enkechen@cisco.com

Pradosh Mohapatra
Cisco Systems
170 W. Tasman Drive Dr.
San Jose, CA 95134
USA
US

Email: pmohapat@cisco.com

Hannes Gredler
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US

Email: hannes@juniper.net