IDR Working Group R. Raszuk, Ed.
Internet-Draft NTT Network Innovations
Intended status: Standards Track B. Decraene, Ed.
Expires: December 19, 2021 Orange
C. Cassar
E. Aman
K. Wang
Juniper Networks
June 17, 2021
BGP Optimal Route Reflection (BGP ORR)
draft-ietf-idr-bgp-optimal-route-reflection-28
Abstract
This document defines an extension to BGP route reflectors. On route
reflectors, BGP route selection is modified in order to choose the
best route from the standpoint of their clients, rather than from the
standpoint of the route reflectors. Depending on the scaling and
precision requirements, route selection can be specific for one
client, common for a set of clients or common for all clients of a
route reflector. This solution is particularly applicable in
deployments using centralized route reflectors, where choosing the
best route based on the route reflector's IGP location is suboptimal.
This facilitates, for example, best exit point policy (hot potato
routing).
The solution relies upon all route reflectors learning all paths
which are eligible for consideration. BGP Route Selection is
performed in the route reflectors based on the IGP cost from
configured locations in the link state IGP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 19, 2021.
Copyright Notice
Copyright (c) 2021 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
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Modifications to BGP Route Selection . . . . . . . . . . . . 4
3.1. Route Selection from a different IGP location . . . . . . 5
3.1.1. Restriction when BGP next hop is a BGP route . . . . 6
3.2. Multiple Route Selections . . . . . . . . . . . . . . . . 6
4. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
However, in some situations, this method suffers from non-optimal
path selection.
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[RFC4456] asserts that, because the 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
Internal BGP (IBGP) mesh approach." One practical implication of
this fact 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 Autonomous System (AS) exit 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 exit point that is
optimal for the route reflector - not necessarily the one that is
optimal for its 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 often been deployed in
the forwarding path and carefully placed on the Point of Presence
(POP) to core boundaries.
The evolving model of intra-domain network design has enabled
deployments of route reflectors outside the forwarding path.
Initially this model was only employed for new services, e.g., IP
VPNs [RFC4364], however it has been gradually extended to other BGP
services, including the IPv4 and IPv6 Internet. In such
environments, hot potato routing policy remains desirable.
Route reflectors outside 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
BGP Route Selection: A route reflector with knowledge of multiple
paths for a given route will typically pick its best path and only
advertise that best path to its clients. If the best path for a
route is selected on the basis of an IGP tie-break, the path
advertised will be the exit point closest to the route reflector.
However, the clients are 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.
In addition, there are deployment scenarios where service providers
want to have more control in choosing the exit points for clients
based on other factors, such as traffic type, traffic load, etc.
This further complicates the issue and makes it less likely for the
route reflector to select the best path from the client's
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perspective. 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.
2. Terminology
This memo makes use of the terms defined in [RFC4271] and [RFC4456].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Modifications to BGP Route Selection
The core of this solution is the ability for an operator to specify
the IGP location for which the route reflector calculates interior
cost for the NEXT_HOP. The IGP location is defined as a node in the
IGP topology, it is identified by an IP address of this node (e.g., a
loopback address), and may be configured on a per route reflector
basis, per set of clients, or per client basis. Such configuration
will allow the route reflector to select and distribute to a given
set of clients routes with shortest distance to the next hops from
the position of the selected IGP location. This provides for freedom
of route reflector physical location, and allows transient or
permanent migration of this network control plane function to an
arbitrary location with no impact to IP transit.
The choice of specific granularity (route reflector, set of clients,
or client) is configured by the network operator. An implementation
is considered compliant with this document if it supports at least
one such grouping category.
For purposes of route selection, the perspective of a client can
differ from that of a route reflector or another client in two
distinct ways:
o it has a different position in the IGP topology,
o it can have a different routing policy.
These factors correspond to the issues described earlier.
This document defines, for BGP Route Reflectors [RFC4456], two
changes to the BGP Route Selection algorithm:
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o The first change, introduced in Section 3.1, is related to the IGP
cost to the BGP Next Hop in the BGP decision process. The change
consists of using the IGP cost from a different IGP location than
the route reflector itself.
o The second change, introduced in Section 3.2, is to extend the
granularity of the BGP decision process, to allow for running
multiple decisions processes using different perspective or
policies.
A significant advantage of these approaches is that the route
reflector clients do not need to be modified.
3.1. Route Selection from a different IGP location
In this approach, optimal refers to the decision where the interior
cost of a route is determined during step e) of [RFC4271] section
9.1.2.2 "Breaking Ties (Phase 2)". It does not apply to path
selection preference based on other policy steps and provisions.
In addition to the change specified in [RFC4456] section 9, [RFC4271]
section 9.1.2.2 is modified as follows.
The below text in step e)
e) Remove from consideration any routes with less-preferred
interior cost. The interior cost of a route is determined by
calculating the metric to the NEXT_HOP for the route using the
Routing Table.
...is replaced by this new text:
e) Remove from consideration any routes with less-preferred
interior cost. The interior cost of a route is determined by
calculating the metric from the selected IGP location to the
NEXT_HOP for the route using the shortest IGP path tree rooted at
the selected IGP location.
In order to be able to compute the shortest path tree rooted at the
selected IGP locations, knowledge of the IGP topology for the area/
level that includes each of those locations is needed. This
knowledge can be gained with the use of the link state IGP such as
IS-IS [ISO10589] or OSPF [RFC2328] [RFC5340] or via BGP-LS [RFC7752].
When specifying logical location of a route reflector for a group of
clients one or more backup IGP locations SHOULD be allowed to be
specified for redundancy. Further deployment considerations are
discussed in Section 4.
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3.1.1. Restriction when BGP next hop is a BGP route
In situations where the BGP next hop is a BGP route itself, the IGP
metric of a route used for its resolution SHOULD be the final IGP
cost to reach such next hop. Implementations which cannot inform BGP
of the final IGP metric to a recursive next hop MUST treat such paths
as least preferred during next hop metric comparison. However, such
paths MUST still be considered valid for BGP Phase 2 Route Selection.
3.2. Multiple Route Selections
BGP Route Reflector as per [RFC4456] runs a single BGP Decision
Process. Optimal route reflection may require multiple BGP Decision
Processes or subsets of the Decision Process in order to consider
different IGP locations or BGP policies for different sets of
clients. This is very similar to what is defined in [RFC7947]
section 2.3.2.1.
If the required routing optimization is limited to the IGP cost to
the BGP Next-Hop, only step e) and subsequent steps as defined in
[RFC4271] section 9.1.2.2, needs to be run multiple times.
If the routing optimization requires the use of different BGP
policies for different sets of clients, a larger part of the decision
process needs to be run multiple times, up to the whole decision
process as defined in section 9.1 of [RFC4271]. This is for example
the case when there is a need to use different policies to compute
different degree of preference during Phase 1. This is needed for
use cases involving traffic engineering or dedicating certain exit
points for certain clients. In the latter case, the user may specify
and apply a general policy on the route reflector for a set of
clients. Regular path selection, including IGP perspective for a set
of clients as per Section 3.1, is then applied to the candidate paths
to select the final paths to advertise to the clients.
A route reflector can implement either or both of the modifications
in order to allow it to choose the best path for its clients that the
clients themselves would have chosen given the same set of candidate
paths.
4. Deployment Considerations
BGP Optimal Route Reflection provides a model for integrating the
client perspective into the BGP Route Selection decision function for
route reflectors. More specifically, the choice of BGP path takes
into account either the IGP cost between the client and the NEXT_HOP
(rather than the IGP cost from the route reflector to the NEXT_HOP)
or other user configured policies.
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The achievement of optimal routing between clients of different
clusters relies upon all route reflectors learning all paths that are
eligible for consideration. In order to satisfy this requirement,
BGP add-path [RFC7911] needs to be deployed between route reflectors.
This solution can be deployed in traditional hop-by-hop forwarding
networks as well as in end-to-end tunneled environments. To avoid
routing loops in networks with multiple route reflectors and hop-by-
hop forwarding without encapsulation, it is essential that the
network topology be carefully considered in designing a route
reflection topology (see also Section 11 of [RFC4456]).
As discussed in section 11 of [RFC4456], the IGP locations of BGP
route reflectors is important and has routing implications. This
equally applies to the choice of the IGP locations configured on
optimal route reflectors. If a backup location is provided, it is
used when the primary IGP location disappears from the IGP (i.e.
fails). Just like the failure of a RR [RFC4456], it may result in
changing the paths selected and advertised to the clients and in
general the post-failure paths are expected to be less optimal. This
is dependent on the IGP topologies and the IGP distance between the
primary and the backup IGP locations: the smaller the distance the
smaller the potential impact.
After selecting suitable IGP locations, an operator may let one or
multiple route reflectors handle route selection for all of them.
The operator may alternatively deploy one or multiple route reflector
for each IGP location or create any design in between. This choice
may depend on operational model (centralized vs per region),
acceptable blast radius in case of failure, acceptable number of IBGP
sessions for the mesh between the route reflectors, performance and
configuration granularity of the equipment.
With this approach, an ISP can effect a hot potato routing policy
even if route reflection has been moved out of the forwarding plane,
and hop-by-hop forwarding has been replaced by end-to-end MPLS or IP
encapsulation. Compared with a deployment of ADD-PATH on all
routers, BGP Optimal Route Reflection (ORR) reduces the amount of
state which needs to be pushed to the edge of the network in order to
perform hot potato routing.
Modifying the IGP location of BGP ORR does not interfere with
policies enforced before IGP tie-breaking (step e) of [RFC4271]
section 9.1.2.2 in the BGP Decision Process.
Calculating routes for different IGP locations requires multiple
Shortest Path First (SPF) calculations and multiple (subsets of) BGP
Decision Processes, which requires more computing resources. This
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document allows for different granularity such as one Decision
Process per route reflector, per set of clients or per client. A
more fine-grained granularity may translate into more optimal hot
potato routing at the cost of more computing power. Selecting to
configure an IGP location per client has the highest precision as
each client can be associated with their ideal (own) IGP location.
However, doing so may have an impact on the performance (as explained
above). Using an IGP location per set of clients implies a loss of
precision, but reduces the impact on the performance of the route
reflector. Similarly, if an IGP location is selected for the whole
routing instance, the lowest precision is achieved, but the
performance impact is minimal. In the last mode of operation both
precision as well as perfomance metrics are equal to same metrics
when using route reflection as described in [RFC4456] without ORR
extension. The ability to run fine-grained computations depends on
the platform/hardware deployed, the number of clients, the number of
BGP routes and the size of the IGP topology. In essence, sizing
considerations are similar to the deployments of BGP Route Reflector.
5. Security Considerations
This extension provides a new metric value using additional
information for computing routes for BGP route reflectors. While any
improperly used metric value could impact the resiliency of the
network, this extension does not change the underlying security
issues inherent in the existing IBGP per [RFC4456].
This document does not introduce requirements for any new protection
measures.
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, Jon
Mitchell, John Scudder, Jeff Haas, Martin Djernaes, Daniele
Ceccarelli, Kieran Milne, Job Snijders, Randy Bush, Alvaro Retana,
Francesca Palombini, Benjamin Kaduk, Zaheduzzaman Sarker, Lars
Eggert, Murray Kucherawy, Tom Petch and Nick Hilliard for their
valuable input.
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8. Contributors
Following persons substantially contributed to the current format of
the document:
Stephane Litkowski
Cisco System
slitkows.ietf@gmail.com
Adam Chappell
GTT Communications, Inc.
Aspira Business Centre
Bucharova 2928/14a
158 00 Prague 13 Stodulky
Czech Republic
adam.chappell@gtt.net
9. References
9.1. 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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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9.2. Informative References
[ISO10589]
International Organization for Standardization,
"Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, Nov 2002.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC7947] Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
"Internet Exchange BGP Route Server", RFC 7947,
DOI 10.17487/RFC7947, September 2016,
<https://www.rfc-editor.org/info/rfc7947>.
Authors' Addresses
Robert Raszuk (editor)
NTT Network Innovations
Email: robert@raszuk.net
Bruno Decraene (editor)
Orange
Email: bruno.decraene@orange.com
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Christian Cassar
Email: cassar.christian@gmail.com
Erik Aman
Email: erik.aman@aman.se
Kevin Wang
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: kfwang@juniper.net
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