Network Working Group D. Walton
Internet-Draft Cumulus Networks
Intended status: Standards Track A. Retana
Expires: February 9, 2017 E. Chen
Cisco Systems, Inc.
J. Scudder
Juniper Networks
August 8, 2016
BGP Persistent Route Oscillation Solutions
draft-ietf-idr-route-oscillation-stop-04
Abstract
The routing information reduction by BGP Route Reflection or
Confederation can result in persistent internal BGP route
oscillations with certain routing setup 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
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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 . . . . . . . . . . . . . . . . . . . . 4
5.2. Confederation . . . . . . . . . . . . . . . . . . . . . . 5
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Why the Group Best Paths Are Adequate? . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
As documented in [RFC3345], the routing information reduction by BGP
Route Reflection [RFC4456] or BGP Confederation [RFC5065] can result
in persistent IBGP route oscillations with certain routing setup and
network topologies. Except for a couple artificially engineered
network topologies, the MULTI_EXIT_DISC attribute (MED) [RFC4271] has
played a pivotal role in virtually all of the 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 the MED-induced route oscillations and to
achieve consistent routing in a network, a route reflector or a
confederation 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 the 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,
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which is a subset of the first one, involves the neighbor-AS based
Group Best Paths, and would be sufficient to eliminate the MED-
induced route oscillations (subject to certain commonly adopted
topological constraints).
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 the 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.
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 which 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 condition is outside the scope of this document.
4. Advertise the Group Best Paths
The term neighbor-AS for a route refers to the neighboring 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 7.2 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
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organize the paths for an address prefix into groups according to
their respective neighbor-AS's, and 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 generally recommended ([RFC4456], [RFC5065]) and widely adopted
practice, 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 the 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 the 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.
5.1. Route Reflection
For a particular <AFI, 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
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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.
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 which 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
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Group Best Paths or Available Paths, which would involve only minor
software changes.
It should be emphasized that in order to eliminate the 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 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-AS's for a particular address prefix. The
additional states for an address prefix would also be per neighbor-AS
based rather than per path based. The number of the neighbor-AS's
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 which are affected by MED-induced
route oscillation in a network carrying a large number of alternate
paths.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This extension to BGP does not change the underlying security issues
inherent in the existing BGP [RFC4271].
9. 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.
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10. References
10.1. 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>.
10.2. 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>.
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, MED)
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comparisons [RFC4271] would contribute to the 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, 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 constrain 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, MED) comparisons
among all the paths in the network, the Group Best Path (when exists)
for a route reflector would be from another cluster. Clearly the
advertise 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.
Authors' Addresses
Daniel Walton
Cumulus Networks
140C S. Whisman Rd.
Mountain View, CA 94041
USA
Email: dwalton@cumulusnetworks.com
Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Email: aretana@cisco.com
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Enke Chen
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA
Email: enkechen@cisco.com
John Scudder
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: jgs@juniper.net
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