IDR Working Group R. Raszuk
Internet-Draft Mirantis Inc.
Intended status: Standards Track C. Cassar
Expires: January 3, 2016 Cisco Systems
E. Aman
TeliaSonera
B. Decraene
S. Litkowski
Orange
July 2, 2015
BGP Optimal Route Reflection (BGP-ORR)
draft-ietf-idr-bgp-optimal-route-reflection-10
Abstract
This document proposes a solution for BGP route reflectors to
facilitate the application of closest exit point policy (hot potato
routing) without requiring further state or any new features to be
placed on the RR clients. This solution is primarily applicable in
deployments using centralized route reflectors.
The solution relies upon all route reflectors learning all paths
which are eligible for consideration for hot potato routing. Best
path selection is performed in each route reflector based on a
configured virtual location in the IGP. The location can be the same
for all clients or different per peer/update group or per neighbour.
Status of This Memo
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This Internet-Draft will expire on January 3, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 2
1.2. Existing/Alternative Solutions . . . . . . . . . . . . . 3
2. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 4
3. CPU and Memory Scalability . . . . . . . . . . . . . . . . . 5
4. Advantages and Deployment Considerations . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
There are three types of BGP deployments within Autonomous Systems
today: full mesh, confederations and route reflection. BGP route
reflection [RFC4456] is the most popular way to distribute BGP routes
between BGP speakers belonging to the same Autonomous System. In
some situations, this method suffers from non-optimal path selection.
1.1. Problem Statement
[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
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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 - not necessarily the one closest to
the RR clients.
Section 11 of [RFC4456] describes a deployment approach and a set of
constraints which, if satisfied, would result in the deployment of
route reflection yielding the same results as the iBGP full mesh
approach. This deployment approach makes route reflection compatible
with the application of hot potato routing policy. In accordance
with these design rules, route reflectors have traditionally often
been deployed in the forwarding path and carefully placed on the POP
to core boundaries.
The evolving model of intra-domain network design has enabled
deployments of route reflectors outside of the forwarding path.
Initially this model was only employed for new address families, e.g.
L3VPNs and L2VPNs. This model has been gradually extended to other
BGP address families including IPv4 and IPv6 Internet using either
native routing or 6PE. In such environments, hot potato routing
policy remains desirable.
Route reflectors outside of the forwarding path 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 pick its best path and only
advertise that best path to its clients. If the best path for a
prefix is selected on the basis of an IGP tie break, the path
advertised will be the exit point closest to the route reflector.
But the clients will be in a different place in the network topology
than the route reflector. In networks where the route reflectors are
not in the forwarding path, 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 had considered the same set of candidate paths
as the route reflector. The path chosen by the client would have
guaranteed the lowest cost and delay trajectory through the network.
1.2. Existing/Alternative Solutions
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
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using attributes otherwise considered less important than IGP cost to
the BGP nexthop.
One possible valid solution or workaround to the best path selection
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 edge routers. Many
networks receive full Internet routing information in a large number
of locations. This could easily result in tens of paths for each
prefix that would need to be distributed to clients.
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
The core of the proposed solution is the ability for an operator to
specify on a per route reflector basis or per peer/update group basis
or per neighbour basis the virtual IGP location placement of the
route reflector. This enables having a given group of clients to
receive routes with optimal distance to the next hops from the
position of the configured virtual IGP location. This also provides
freedom on route reflector location and allows transient or permanent
migration of such network control plane function to optimal location.
The choice of specific granularity is left to the implementation
decision. An implementation is considered compliant with the
document if it supports at least one listed grouping of virtual IGP
placement.
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.
This document does not apply to path selection preference based other
policy steps and provisions.
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The solution relies 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-PATH/diverse paths may need to be deployed
between route reflectors.
A significant advantage of this approach is that the RR clients do
not need to run new software or hardware.
The computation of the virtual IGP location with any of the above
described granularity is outside of the scope of this document. The
operator may configure it manually, implementation may automate it
based on specified heuristic or it can be computed centrally and
configured by an external system.
The solution does not require any BGP or IGP protocol changes as
required changes are contained within the RR implementation.
The solution applies to NLRIs of all address families which can be
route reflected.
3. CPU and Memory Scalability
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 resources.
However SPF tree generation code is now implemented efficiently in a
number of implementations, and therefore this is not expected to be a
major drawback. The number of SPTs computed is expected to be of the
order of the number of clients of an RR whenever a topology change is
detected. Advanced optimizations like partial and incremental SPF
may also be exploited. The number of SPTs computed is expected to be
higher but comparable to some existing deployed features such as
(Remote) Loop Free Alternate which computes a (r)SPT per IGP
neighbor.
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
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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.
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 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 a BGP control
plane with centralized route reflection without compromising an
operator's closest exit operational principle. This enables edge-to-
edge LSP/IP encapsulation for traffic to IPv4 and IPv6 prefixes.
Regarding the 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.
5. Security Considerations
No new security issues are introduced to the BGP protocol by this
specification.
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6. IANA Considerations
This document does not request any IANA allocations.
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.
[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.
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[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
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
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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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