Internet Engineering Task Force (IETF) R. Raszuk, Ed.

Internet Engineering Task Force (IETF) R. Raszuk, Ed.
Request for Comments: 9107 NTT Network Innovations
Category: Standards Track B. Decraene, Ed.
ISSN: 2070-1721 Orange
C. Cassar

E. Åman

K. Wang
Juniper Networks
August 2021

BGP Optimal Route Reflection (BGP ORR)

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 themselves. 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, a "best exit point"
policy ("hot potato routing").

The solution relies upon all route reflectors learning all paths that
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 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
https://www.rfc-editor.org/info/rfc9107.

Copyright Notice

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Table of Contents

1. Introduction
2. Terminology
3. Modifications to BGP Route Selection
3.1. Route Selection from a Different IGP Location
3.1.1. Restriction when the BGP Next Hop Is a BGP Route
3.2. Multiple Route Selections
4. Deployment Considerations
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

There are three types of BGP deployments within Autonomous Systems
(ASes) 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 AS. However, in
some situations, this method suffers from non-optimal path selection.

[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 IBGP
mesh approach." ("IBGP" stands for "Internal BGP".) 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 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 that, 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 a 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 boundaries between
the Point of Presence (POP) and the network core.

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, a hot potato routing policy remains desirable.

Route reflectors outside the forwarding path can be placed on the
boundaries between the POP and the network core, 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
perspective. It follows that the best path chosen by the route
reflector is not necessarily the same as the path that 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 to 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 it may be configured on a per-route-reflector
basis, per set of clients, or on a per-client basis. Such
configuration will allow the route reflector to select and distribute
to a given set of clients routes with the shortest distance to the
next hops from the position of the selected IGP location. This
provides for freedom related to the route reflector's physical
location and allows transient or permanent migration of this network
control plane function to an arbitrary location with no impact on 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:

* It has a different position in the IGP topology.

* 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:

* 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.

* The second change, introduced in Section 3.2, is to extend the
granularity of the BGP Decision Process, to allow for running
multiple Decision Processes using different perspectives or
policies.

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.

A significant advantage of these approaches is that the route
reflector's 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 Section 9.1.2.2
("Breaking Ties (Phase 2)") of [RFC4271]. It does not apply to path
selection preference based on other policy steps and provisions.

In addition to the change specified in Section 9 of [RFC4456], the
text in step e) in Section 9.1.2.2 of [RFC4271] is modified as
follows.

RFC 4271 reads:

| 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.

This document modifies this text to read:

| 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 the Border
Gateway Protocol - Link State (BGP-LS) [RFC7752]. When specifying
the 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.

3.1.1. Restriction when the 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 a next hop. Implementations that cannot inform
BGP of the final IGP metric to a recursive next hop MUST treat such
paths as least preferred during next-hop metric comparisons.
However, such paths MUST still be considered valid for BGP Phase 2
route selection.

3.2. Multiple Route Selections

A BGP route reflector as per [RFC4456] runs a single BGP Decision
Process. BGP Optimal Route Reflection (BGP ORR) 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 need 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 degrees 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
perspectives 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.

4. Deployment Considerations

BGP ORR provides a model for integrating the client's perspective
into the BGP route selection Decision Process 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.

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 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 are important and have 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 route reflector [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 backup IGP locations: the smaller
the distance, the smaller the potential impact.

After selecting N suitable IGP locations, an operator can choose to
enable route selection for all of them on all or on a subset of their
route reflectors. The operator may alternatively deploy single or
multiple (backup case) route reflectors for each IGP location or
create any design in between. This choice may depend on the
operational model (centralized vs. per region), an acceptable blast
radius in the case of failure, an 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 ORR reduces the amount of state that 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. This scenario calls for more computing
resources. This 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.
Choosing 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 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 impact on performance is minimal. In the last mode of operation
(where an IGP location is selected for the whole routing instance),
both precision and performance metrics are equal to route reflection
as described in [RFC4456]. 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 reflectors.

5. Security Considerations

The extension specified in this document 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 has no IANA actions.

7. References

7.1. 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>.

7.2. 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, November 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>.

Acknowledgments

The 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 Djernæs, 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.

Contributors

The following persons contributed substantially to the current format
of the document:

Stephane Litkowski
Cisco Systems

Email: [email protected]

Adam Chappell
GTT Communications, Inc.
Aspira Business Centre
Bucharova 2928/14a
158 00 Prague 13 Stodůlky
Czech Republic

Email: [email protected]

Authors' Addresses

Robert Raszuk (editor)
NTT Network Innovations

Email: [email protected]

Bruno Decraene (editor)
Orange

Email: [email protected]

Christian Cassar

Email: [email protected]

Erik Åman

Email: [email protected]

Kevin Wang
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
United States of America

Email: [email protected]