Internet Engineering Task Force (IETF) D. Walton

Internet Engineering Task Force (IETF) D. Walton
Request for Comments: 7964 Cumulus Networks
Category: Standards Track A. Retana
ISSN: 2070-1721 E. Chen
Cisco Systems, Inc.
J. Scudder
Juniper Networks
September 2016

Solutions for BGP Persistent Route Oscillation

Abstract

Routing information reduction by BGP Route Reflection or
Confederation can result in persistent internal BGP route
oscillations with certain routing setups and network topologies.
This document specifies two sets of additional paths that can be used
to eliminate these route oscillations in a network.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7964.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Advertise All the Available Paths . . . . . . . . . . . . . . 3
4. Advertise the Group Best Paths . . . . . . . . . . . . . . . 3
5. Route Reflection and Confederation . . . . . . . . . . . . . 4
5.1. Route Reflection . . . . . . . . . . . . . . . . . . . . 5
5.2. Confederation . . . . . . . . . . . . . . . . . . . . . . 5
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Why the Group Best Paths Are Adequate . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9

1. Introduction

As documented in [RFC3345], routing information reduction by BGP
Route Reflection [RFC4456] or BGP Confederation [RFC5065] can result
in persistent Internal BGP (IBGP) route oscillations with certain
routing setups and network topologies. Except for a couple of
artificially engineered network topologies, the MULTI_EXIT_DISC (MED)
attribute [RFC4271] has played a pivotal role in virtually all known
persistent IBGP route oscillations. For the sake of brevity, we use
the term "MED-induced route oscillation" hereafter to refer to a
persistent IBGP route oscillation in which the MED plays a role.

In order to eliminate MED-induced route oscillations and to achieve
consistent routing in a network, a route reflector or a confederation
Autonomous System Border Router (ASBR) needs to advertise more than
just the best path for an address prefix. Our goal is to identify
the necessary set of paths for an address prefix that needs to be
advertised by a route reflector or a confederation ASBR to prevent
the condition.

In this document, we describe two sets of paths for an address prefix
that can be advertised by a BGP route reflector or confederation ASBR
to eliminate MED-induced route oscillations in a network. The first
set involves all the available paths, and would achieve the same
routing consistency as the full IBGP mesh. The second set, which is
a subset of the first one, involves the neighbor-AS-based Group Best
Paths, and would be sufficient to eliminate MED-induced route
oscillations (subject to certain commonly adopted topological
constraints).

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These paths can be advertised using the mechanism described in
ADD-PATH [RFC7911] for advertising multiple paths. No other
assumptions in functionality beyond the base BGP specification
[RFC4271] are made.

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 [RFC2119].

3. Advertise All the Available Paths

Observe that in a network that maintains a full IBGP mesh, all the
BGP speakers have consistent and equivalent routing information.
Such a network is thus free of MED-induced route oscillations and
other routing inconsistencies such as forwarding loops.

Therefore, one approach is to allow a route reflector or a
confederation ASBR to advertise all the available paths for an
address prefix. Clearly this approach would yield the same amount of
routing information and achieve the same routing consistency as the
full IBGP mesh in a network. In this document, "Available Paths"
refers to the advertisement of all the available paths.

This approach can be implemented using the mechanism described in
ADD-PATH [RFC7911] for advertising multiple paths for certain
prefixes.

For the sake of scalability, the advertisement of multiple paths
should be limited to those prefixes that are affected by MED-induced
route oscillation in a network carrying a large number of alternate
paths. A detailed description of how these oscillations can occur
can be found in [RFC3345]; the description of how a node would
locally detect such conditions is outside the scope of this document.

4. Advertise the Group Best Paths

The term "neighbor-AS" for a route refers to the neighboring
autonomous system (AS) from which the route was received. The
calculation of the neighbor-AS is specified in Section 9.1.2.2 of
[RFC4271], and Section 5.3 of [RFC5065]. By definition, the MED is
comparable only among routes with the same neighbor-AS. Thus, the
route selection procedures specified in [RFC4271] would conceptually
involve two steps: first, organize the paths for an address prefix
into groups according to their respective neighbor-ASes, and

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RFC 7964 BGP Oscillation Solutions September 2016

calculate the most preferred one (termed "Group Best Path") for each
of the groups; then, calculate the overall best path among all the
Group Best Paths.

As a practice that is generally recommended (in [RFC4456] and
[RFC5065]) and widely adopted, a route reflection cluster or a
confederation sub-AS should be designed such that BGP routes from
within the cluster (or confederation sub-AS) are preferred over
routes from other clusters (or confederation sub-AS) when the
decision is based on the IGP cost to the BGP NEXT_HOP. This is
typically done by setting IGP metrics for links within a cluster (or
confederation sub-AS) to be much smaller than the IGP metrics for the
links between the clusters (or confederation sub-AS). This practice
helps achieve consistent routing within a route reflection cluster or
a confederation sub-AS.

When the aforementioned practice for devising a route reflection
cluster or confederation sub-AS is followed in a network, we claim
that the advertisement of all the Group Best Paths by a route
reflector or a confederation ASBR is sufficient to eliminate
MED-induced route oscillations in the network. This claim is
validated in Appendix A.

Note that a Group Best Path for an address prefix can be identified
by the combination of the address prefix and the neighbor-AS. Thus,
this approach can be implemented using the mechanism described in
ADD-PATH [RFC7911] for advertising multiple paths, and in this case,
the neighbor-AS of a path may be used as the path identifier of the
path.

It should be noted that the approach of advertising the Group Best
Paths requires certain topological constraints to be satisfied in
order to eliminate MED-induced route oscillation. Specific
topological considerations are described in [RFC3345].

5. Route Reflection and Confederation

To allow a route reflector or a confederation ASBR to advertise
either the Available Paths or Group Best Paths using the mechanism
described in ADD-PATH [RFC7911], the following revisions are proposed
for BGP Route Reflection and BGP Confederation.

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5.1. Route Reflection

For a particular <Address Family Identifier (AFI), Subsequent Address
Family (SAFI)>, a route reflector MUST include the <AFI, SAFI> with
the "Send/Receive" field set to 2 (send multiple paths) or
3 (send/receive multiple paths) in the ADD-PATH Capability [RFC7911]
advertised to an IBGP peer. When the ADD-PATH Capability is also
received from the IBGP peer with the "Send/Receive" field set to 1
(receive multiple paths) or 3 (send/receive multiple paths) for the
same <AFI, SAFI>, then the following procedures apply:

If the peer is a route reflection client, the route reflector MUST
advertise to the peer the Group Best Paths (or the Available Paths)
received from its non-client IBGP peers. The route reflector MAY
also advertise to the peer the Group Best Paths (or the Available
Paths) received from its clients.

If the peer is a non-client, the route reflector MUST advertise to
the peer the Group Best Paths (or the Available Paths) received from
its clients.

5.2. Confederation

For a particular <AFI, SAFI>, a confederation ASBR MUST include the
<AFI, SAFI> with the "Send/Receive" field set to 2 (send multiple
paths) or 3 (send/receive multiple paths) in the ADD-PATH Capability
[RFC7911] advertised to an IBGP peer, and to a confederation external
peer. When the ADD-PATH Capability is also received from the IBGP
peer or the confederation-external peer with the "Send/Receive" field
set to 1 (receive multiple paths) or 3 (send/receive multiple paths)
for the same <AFI, SAFI>, then the following procedures apply:

If the peer is internal, the confederation ASBR MUST advertise to the
peer the Group Best Paths (or the Available Paths) received from its
confederation-external peers.

If the peer is confederation-external, the confederation ASBR MUST
advertise to the peer the Group Best Paths (or the Available Paths)
received from its IBGP peers.

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6. Deployment Considerations

Some route oscillations, once detected, can be eliminated by simple
configuration workarounds. As carrying additional paths impacts the
memory usage and routing convergence in a network, it is recommended
that the impact be evaluated and the approach of using a
configuration workaround be considered in deciding whether to deploy
the proposed mechanism in a network. In addition, the advertisement
of multiple paths should be limited to those prefixes that are
affected by MED-induced route oscillation.

While the route reflectors or confederation ASBRs in a network need
to advertise the Group Best Paths or Available Paths, the vast
majority of the BGP speakers in the network only need to receive the
Group Best Paths or Available Paths, which would involve only minor
software changes.

It should be emphasized that, in order to eliminate MED-induced route
oscillations in a network using the approach of advertising the Group
Best Paths, the recommended practice for devising a route reflection
cluster or confederation sub-AS with respect to the IGP metrics
([RFC4456] [RFC5065]) should be followed.

It is expected that the approach of advertising the Group Best Paths
would be adequate to achieve consistent routing for the vast majority
of the networks. For a network that has a large number of alternate
paths, the approach should be a good choice as the number of paths
advertised by a reflector or a confederation ASBR is bounded by the
number of the neighbor-ASes for a particular address prefix. The
additional states for an address prefix would also be per neighbor-AS
rather than per path. The number of neighbor-ASes for a particular
address prefix is typically small because of the limited number of
upstream providers for a customer and the nature of advertising only
customer routes at the inter-exchange points.

The approach of advertising the Group Best Paths, however, may still
be inadequate for certain networks to avoid other routing
inconsistencies such as forwarding loops. The required topological
constraints could also be operationally challenging. In these cases
the approach of advertising the Available Paths may be used, but
should be limited to those prefixes that are affected by MED-induced
route oscillation in a network carrying a large number of alternate
paths.

7. Security Considerations

This extension to BGP does not change the underlying security issues
inherent in the existing BGP [RFC4271].

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8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<http://www.rfc-editor.org/info/rfc4456>.

[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007,
<http://www.rfc-editor.org/info/rfc5065>.

[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<http://www.rfc-editor.org/info/rfc7911>.

8.2. Informative References

[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345,
August 2002, <http://www.rfc-editor.org/info/rfc3345>.

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Appendix A. Why the Group Best Paths Are Adequate

It is assumed that the following common practice is followed. A
route reflection cluster or a confederation sub-AS should be designed
such that the IGP metrics for links within a cluster (or
confederation sub-AS) are much smaller than the IGP metrics for the
links between the clusters (or confederation sub-AS). This practice
helps achieve consistent routing within a route reflection cluster or
a confederation sub-AS.

Observe that in a network that maintains full IBGP mesh only, the
paths that survive the (Local_Pref, AS-PATH Length, Origin, and MED)
comparisons [RFC4271] would contribute to route selection in the
network.

Consider a route reflection cluster that sources one or more paths
that would survive the (Local_Pref, AS-PATH Length, Origin, and MED)
comparisons among all the paths in the network. One of these
surviving paths would be selected as the Group Best Path by the route
reflector in the cluster. Due to the constraint on the IGP metrics
as described previously, this path would remain as the Group Best
Path and would be advertised to all other clusters even after a path
is received from another cluster.

On the other hand, when no path in a route reflection cluster would
survive the (Local_Pref, AS-PATH Length, Origin, and MED) comparisons
among all the paths in the network, the Group Best Path (when it
exists) for a route reflector would be from another cluster.
Clearly, the advertisement of the Group Best Path by the route
reflector to the clients only depends on the paths received from
other clusters.

Therefore, there is no MED-induced route oscillation in the network
as the advertisement of a Group Best Path to a peer does not depend
on the paths received from that peer.

The claim for the confederation can be validated similarly.

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Acknowledgements

We would like to thank David Cook and Naiming Shen for their
contributions to the design and development of the solutions.

Many thanks to Tony Przygienda, Sue Hares, Jon Mitchell, and Paul
Kyzivat for their helpful suggestions.

Authors' Addresses

Daniel Walton
Cumulus Networks
140C S. Whisman Rd.
Mountain View, CA 94041
United States of America

Email: [email protected]

Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
United States of America

Email: [email protected]

Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
United States of America

Email: [email protected]

John Scudder
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
United States of America

Email: [email protected]

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