IDR Working Group R. Raszuk
Internet-Draft Mirantis Inc.
Intended status: Standards Track C. Cassar
Expires: October 28, 2015 Cisco Systems
E. Aman
TeliaSonera
B. Decraene
S. Litkowski
Orange
April 26, 2015
BGP Optimal Route Reflection (BGP-ORR)
draft-ietf-idr-bgp-optimal-route-reflection-09
Abstract
[RFC4456] asserts that, because the Interior Gateway Protocol (IGP)
cost to a given point in the network will vary across routers, "the
route reflection approach may not yield the same route selection
result as that of the full IBGP mesh approach." One practical
implication of this assertion is that the deployment of route
reflection may thwart the ability to achieve hot potato routing. Hot
potato routing attempts to direct traffic to the closest AS egress
point in cases where no higher priority policy dictates otherwise.
As a consequence of the route reflection method, the choice of exit
point for a route reflector and its clients will be the egress point
closest to the route reflector - and not necessarily closest to the
RR clients.
Section 11 of [RFC4456] describes a deployment approach and a set of
constraints which, if satsified, would result in the deployment of
route reflection yielding the same results as the iBGP full mesh
approach. Such a deployment approach would make route reflection
compatible with the application of hot potato routing policy.
As networks evolved to accommodate architectural requirements of new
services, tunneled (LSP/IP tunneling) networks with centralized route
reflectors became commonplace. This is one type of common deployment
where it would be impractical to satisfy the constraints described in
Section 11 of [RFC4456]. Yet, in such an environment, hot potato
routing policy remains desirable.
This document proposes a new solution which can be deployed to
facilitate the application of closest exit point policy in
centralized route reflection deployments.
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Status of This Memo
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This Internet-Draft will expire on October 28, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed solution . . . . . . . . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Advantages and deployment considerations . . . . . . . . . . 6
5. Security considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
There are three types of BGP deployments within Autonomous Systems
today: full mesh, confederations and route reflection.
BGP route reflection is the most popular way to distribute BGP routes
between BGP speakers belonging to the same administrative domain.
Traditionally route reflectors have been deployed in the forwarding
path and carefully placed on the POP to core boundaries. That model
of BGP route reflector placement has started to evolve. The
placement of route reflectors outside the forwarding path was
triggered by applications which required traffic to be tunneled from
AS ingress PE to egress PE: for example L3VPN.
This evolving model of intra-domain network design has enabled
deployments of centralized route reflectors. Initially this model
was only employed for new address families e.g. L3VPNs, L2VPNs etc
With edge to edge MPLS or IP encapsulation also being used to carry
internet traffic, this model has been gradually extended to other BGP
address families including IPv4 and IPv6 Internet routing. This is
also applicable to new services achieved with BGP as control plane
for example 6PE.
Such centralized route reflectors can be placed on the POP to core
boundaries, but they are often placed in arbitrary locations in the
core of large networks.
Such deployments suffer from a critical drawback in the context of
best path selection. A route reflector with knowledge of multiple
paths for a given prefix will typically (unless other techniques like
add paths are in use) pick the best path and only advertise that best
path to the the route reflector clients. If the best path for a
prefix is selected on the basis of an IGP tie break, the best path
advertised from the route reflector to its clients will be the exit
point closest to the route reflector. But route reflector clients
will be in a place in the network topology which is different from
the route reflector. In networks with centralized route reflectors,
this difference will be even more acute. It follows that the best
path chosen by the route reflector is not necessarily the same as the
path which would have been chosen by the client if the client
considered the same set of candidate paths as the route reflector.
Furthermore, the path chosen by the client might have been a better
path from that chosen by the route reflector for traffic entering the
network at the client. The path chosen by the client would have
guaranteed the lowest cost and delay trajectory through the network.
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Route reflector clients switch packets using routing information
learnt from route reflectors which are not on the forwarding path of
the packet through the network even in the absence of end-to-end
encapsulation. In those cases the path chosen as best and propagated
to the clients will often not be the optimal path chosen by the
client given all available paths.
Eliminating the IGP distance to the BGP nexthop as a tie breaker on
centralized route reflectors does not address the issue. Ignoring
IGP distance to the BGP next hop results in the tie breaking
procedure contributing the best path by differentiating between paths
using attributes otherwise considered less important than IGP cost to
the BGP nexthop.
One possible valid solution or workaround to this problem requires
sending all domain external paths from the RR to all its clients.
This approach suffers the significant drawback of pushing a large
amount of BGP state to all the edge routers. In many networks, the
number of EBGP peers over which full Internet routing information is
received would correlate directly to the number of paths present in
each ASBR. This could easily result in tens of paths for each
prefix.
Notwithstanding this drawback, there are a number of reasons for
sending more than just the single best path to the clients. Improved
path diversity at the edge is a requirement for fast connectivity
restoration, and a requirement for effective BGP level load
balancing.
In practical terms, add/diverse path deployments are expected to
result in the distribution of 2, 3 or n (where n is a small number)
'good' paths rather than all domain external paths. While the route
reflector chooses one set of n paths and distributes those same n
paths to all its route reflector clients, those n paths may not be
the right n paths for all clients. In the context of the problem
described above, those n paths will not necessarily include the
closest egress point out of the network for each route reflector
client. The mechanisms proposed in this document are likely to be
complementary to mechanisms aimed at improving path diversity.
2. Proposed solution
This document proposes a simple solution to the problem described
above - overwrite of the default IGP location placement of the route
reflector - which is used for determining cost to the next hop
contained in BGP paths.
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The presented solution makes it possible for route reflector clients
to direct traffic to their closest exit point in hot potato routing
deployments, without requiring further state to be pushed out to the
edge. This solution is primarily applicable in deployments using
centralized route reflectors, which are typically implemented in
devices without a capable forwarding plane or which are being moved
to the NFV enabled cloud.
The solution rely upon all route reflectors learning all paths which
are eligible for consideration for hot potato routing. In order to
satisfy this requirement, path diversity enhancing mechanisms such as
add paths/diverse paths may need to be deployed between route
reflectors.
The core of the proposed solution is the ability for operator to
specify on a per route reflector basis or per peer/update group basis
or per neighbour basis the virtual IGP location placement allowing to
have given group of clients to consider optimal distance to the next
hops from the position of the configured virtual IGP location. The
choice of specific granularity is left to the implementation
decision. Implementation is considered as compliant with the
document if it supports at least one listed grouping of virtual IGP
placement.
The computation of the virtual IGP location with any of the above
described granularity is outside of the scope of this document.
Operator may configure it manually, implementation may automate it
based on specified heuristic or it can be computed centrally and
configured by external system.
By optimal we refer in this document to the decision made during best
path selection at the IGP metric to BGP next hop comparison step.
Clearly the overall path selection preference may be chosen based
other policy step and provisions as defined in this document would
not apply.
A significant advantage of this approach is that the RR clients do
not need to run new software or hardware.
3. Discussion
Determining the shortest path and associated cost between any two
arbitrary points in a network based on the IGP topology learned by a
router is expected to add some extra cost in terms of CPU resource.
However SPF tree generation code is now implemented efficiently in a
number of implementations, and therefor this is not expected to be a
major drawback. The number of SPTs computed in the general non-
hierarchical case is expected to be of the order of the number of
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clients of an RR whenever a topology change is detected. Advanced
optimizations like partial and incremental SPF may also be exploited.
By the nature of route reflection, the number of clients can be split
arbitrarily by the deployment of more route reflectors for a given
number of clients. While this is not expected to be necessary in
existing networks with best in class route reflectors available
today, this avenue to scaling up the route reflection infrastructure
would be available. If we consider the overall network wide cost/
benefit factor, the only alternative to achieve the same level of
optimality would require significantly increasing state on the edges
of the network, which, in turn, will consume CPU and memory resources
on all BGP speakers in the network. Building this client perspective
into the route reflectors seems appropriate.
4. Advantages and deployment considerations
The solution described provides a model for integrating the client
perspective into the best path computation for RRs. More
specifically, the choice of BGP path factors in the IGP metric
between the client and the nexthop, rather than the distance from the
RR to the nexthop. The documented method does not require any BGP or
IGP protocol changes as required changes are contained within the RR
implementation.
This solution can be deployed in traditional hop-by-hop forwarding
networks as well as in end-to-end tunneled environments. In the
networks where there are multiple route reflectors and hop-by-hop
forwarding without encapsulation, such optimizations should be
enabled in a consistent way on all route reflectors. Otherwise
clients may receive an inconsistent view of the network and in turn
lead to intra-domain forwarding loops.
With this approach, an ISP can effect a hot potato routing policy
even if route reflection has been moved from the forwarding plane
(example ABR) tothe core and hop-by-hop switching has been replaced
by end to end MPLS or IP encapsulation.
As per above, the approach reduces the amount of state which needs to
be pushed to the edge in order to perform hot potato routing. The
memory and CPU resource required at the edge to provide hot potato
routing using this approach is lower than what would be required in
order to achieve the same level of optimality by pushing and
retaining all available paths (potentially 10s) per each prefix at
the edge.
The proposal allows for a fast and safe transition to BGP control
plane route reflection without compromising an operator's closest
exit operational principle. Hot potato routing is important to most
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ISPs. The inability to perform hot potato routing effectively stops
migrations to centralized route reflection and edge-to-edge LSP/IP
encapsulation for traffic to IPv4 and IPv6 prefixes.
Regarding potential for intra-domain forwarding loops at ASBR level,
this could be solved by enforcing external route preference or by
performing tunnel to external interface switching action on ASBRs.
Regarding client's IGP best-path selection, it should be self evident
that this solution does not interfere with policies enforced above
IGP tie breaking in the BGP best path algorithm.
The solution applies to NLRIs of all address families which can be
route reflected.
5. Security considerations
No new security issues are introduced to the BGP protocol by this
specification.
6. IANA Considerations
IANA is requested to allocate a type code for the Standard BGP
Community to be used for inter cluster propagation of angular
position of the clients.
IANA is requested to allocate a new type code from BGP OPEN Optional
Parameter Types registry to be used for Group_ID propagation.
7. Acknowledgments
Authors would like to thank Keyur Patel, Eric Rosen, Clarence
Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand and Jon
Mitchell for their valuable input.
8. References
8.1. 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.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
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[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, February 2009.
8.2. Informative References
[I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-10 (work in progress), October 2014.
[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996.
[RFC1998] Chen, E. and T. Bates, "An Application of the BGP
Community Attribute in Multi-home Routing", RFC 1998,
August 1996.
[RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114,
RFC 4384, February 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007.
[RFC5283] Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
July 2008.
[RFC5668] Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS
Specific BGP Extended Community", RFC 5668, October 2009.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
[RFC6774] Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
Kumaki, "Distribution of Diverse BGP Paths", RFC 6774,
November 2012.
Authors' Addresses
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Robert Raszuk
Mirantis Inc.
615 National Ave. #100
Mt View, CA 94043
USA
Email: robert@raszuk.net
Christian Cassar
Cisco Systems
10 New Square Park
Bedfont Lakes, FELTHAM TW14 8HA
UK
Email: ccassar@cisco.com
Erik Aman
TeliaSonera
Marbackagatan 11
Farsta SE-123 86
Sweden
Email: erik.aman@teliasonera.com
Bruno Decraene
Orange
38-40 rue du General Leclerc
Issy les Moulineaux cedex 9 92794
France
Email: bruno.decraene@orange.com
Stephane Litkowski
Orange
9 rue du chene germain
Cesson Sevigne 35512
France
Email: stephane.litkowski@orange.com
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