wdiff draft-ietf-idr-bgp-optimal-route-reflection-21.txt draft-ietf-idr-bgp-optimal-route-reflection-22.txt

IDR Working Group R. Raszuk, Ed.
Internet-Draft Bloomberg LP NTT Network Innovations
Intended status: Standards Track C. Cassar
Expires: December 18, 2020 July 19, 2021 Tesla
E. Aman
Telia Company

B. Decraene, Ed.
Orange
K. Wang
Juniper Networks
June 16, 2020
January 15, 2021

BGP Optimal Route Reflection (BGP-ORR)
draft-ietf-idr-bgp-optimal-route-reflection-21
draft-ietf-idr-bgp-optimal-route-reflection-22

Abstract

This document defines an extension to BGP route reflectors. On route
reflectors, BGP route selection is modified in order to choose the
best path for from the standpoint of their clients standpoint, clients, rather than from the
standpoint of the route
reflectors standpoint. reflectors. Multiple type types of granularity
are proposed, from a per client BGP route selection or to a per peer
group, depending on the scaling and precision requirements on route
selection. This solution is particularly applicable in deployments
using centralized route reflectors, where choosing the best route
based on the Route Reflector route reflector 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. Best path selection is
performed in each route reflector based on the IGP cost from a
selected location 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
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This Internet-Draft will expire on December 18, 2020. July 19, 2021.

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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Table of Contents

1. Definitions of Terms Used in This Memo . . . . . . . . . . . 3 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Modifications to BGP Best Path selection . . . . . . . . . . 5
3.1. IGP Based Best Path Selection from a different SPT root IGP location . . . . 6
3.1.1. Restriction when BGP next hop is BGP prefix . . . . . 7
3.2. Multiple Best Path Selections granularity . . . . . . . . . . . . 7
4. Solution Interactions . . . . 7
4. Implementation considerations . . . . . . . . . . . . . . . . 8 7
4.1. IGP and policy based optimal route refresh . . . . . . . 8
4.2. Add-paths plus IGP and policy optimal route refresh . . . 8
4.3. Likely Deployments and need for backup . . . . . . . . . 8 7
5. CPU and Memory Scalability . . . . . . . . . . . . . . . . . 9 8
6. Advantages and Deployment Considerations . . . . . . . . . . 10 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 10
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 11 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 12 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12 11
Appendix A. Appendix: alternative solutions with limited
applicability . . . . . . . . . . . . . . . . . . . 14 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 13

1. Definitions of Terms Used in This Memo

NLRI - Network Layer Reachability Information. Information

RIB - Routing Information Base. Base
AS - Autonomous System number. number

VRF - Virtual Routing and Forwarding instance. instance

PE - Provider Edge router

RR - Route Reflector

POP - Point Of Presence

L3VPN - Layer 3 Virtual Private Networks Network [RFC4364]

6PE - IPv6 Provider Edge Router [RFC4798]

IGP - Interior Gateway Protocol

SPT - Shortest Path Tree

best path - the route chosen by the decision process detailed in
[RFC 4271]
[RFC4271] section 9.1.2 and its subsections

best path computation - the decision process detailed in [RFC 4271] [RFC4271]
section 9.1.2 and its subsections

best path algorithm - the decision process detailed in [RFC 4271] [RFC4271]
section 9.1.2 and its subsections

best path selection - the decision process detailed in [RFC 4271] [RFC4271]
section 9.1.2 and its subsections

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.

2. 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.

[RFC4456] asserts that, because the Interior Gateway Protocol (IGP) 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 best 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 best that is optimal for the route reflector - not
necessarily the one best that is optimal for the route
reflector 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 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, however it 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, 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 which would have
been chosen by the client if the client had considered the same set
of candidate paths as the route reflector.

3. Modifications to BGP Best Path selection

The core of this solution is the ability for an operator to specify
the IGP location for which the route reflector should calculate
routes. This can be done on a per route reflector basis, or per peer/update peer/
update group basis, or per peer basis the IGP location of the route reflector. basis. This core ability enables the
route reflector to send to a given group set of clients routes with
shortest distance to the next hops from the position of the selected
IGP location. This core ability provides for freedom of route reflector physical
location, and allows transient or permanent migration of this network
control plane function to an arbitrary location.

The choice of specific granularity (route reflector basis, peer/
update group basis, reflector, peer/update
group, or peer peer basis) peer) is configured by the network operator. An
implementation is considered compliant with the this document if it
supports at least one listed grouping of IGP location.

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 can, and usually will, have a different position in the IGP
topology, and

o it can have a different routing policy.

These factors correspond to the issues described earlier.

This document defines, on BGP Route Reflectors [RFC4456], two changes
to the BGP Best Path selection algorithm:

o The first change change, introduced in Section 3.1, is related to the IGP
cost to the BGP Next Hop,
which is done in the step e) Hop in the BGP decision process. The change
consists in using the IGP cost from a different source IGP location than
the route reflector itself.

o The second change change, introduced in Section 3.2, is to extend the
granularity of the BGP decision process, to allow for running
multiple decisions process using different perspective 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.

Both modifications rely upon all route reflectors learning all paths
that are eligible for consideration. In order to satisfy this
requirement, path diversity enhancing mechanisms such as add-path may
need to be deployed between route reflectors.

A significant advantage of these approaches is that the route
reflector clients do not need to run new software or hardware.

3.1. IGP Based Best Path Selection from a different SPT root IGP location

In this approach, optimal refers to the decision made during best
path selection at the IGP metric to BGP next hop comparison step.
This approach 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, the BGP
Decision Process Tie Breaking tie-breaking rules ([RFC4271] Sect. section 9.1.2.2) are
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 on at
the selected IGP location.

This extension requires the knowledge of the IGP topology in

In order to be able to compute the shortest path tree rooted on any location and
in particular on at the
selected IGP locations. 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]. If an IGP is used,
the selected IGP location MUST to be within the area/level of the
IGP.

The configuration of the

The configuration of the IGP location is outside of the scope of this
document. The operator may configure it manually, an implementation
may automate it based on heuristics, or it can be computed centrally
and configured by an external system.

This solution does not require any change (BGP or IGP) on the
clients, as all required changes are limited to the route reflector.

This solution applies to NLRIs of all address families, families that can be
route reflected.

3.1.1. Restriction when BGP next hop is BGP prefix

In situations where the BGP next hop is a BGP prefix itself itself, the IGP
metric of a route used for its resolution SHOULD be the final IGP
cost to reach such next hop. Implementations which can not inform
BGP of the final IGP metric to a recursive next hop SHOULD treat such
paths as least preferred during next hop metric comparison. However
such paths SHOULD still be considered valid for best path selection.

3.2. Multiple Best Path Selections granularity

BGP Route Reflector as per [RFC4456] runs the usual a single Best Path
Selection used to compute the node's routing table. This best path
selection. Optimal route reflection may be
suboptimal or even not usuable when the Route Reflector clients has
significantly different IGP locations or BGP policies. In some
cases, there is a need to compute the Best Path Selection with an
increased granularity, such as per peer/update group or per client
basis.

This requires running require calculation of
multiple best path selections or multiple
subset subsets of the best path selection. selection in
order to consider different IGP locations or BGP policies for
different sets of clients.

If the required routing optimization is limited to the IGP cost to
the BGP Next-Hop, which is
typical if the goal is hot potato routing or a routing (more) similar
to the one resulting from an iBGP full mesh between clients, only the step e) as defined [RFC4271] Sect. section 9.1.2.2,
needs to be duplicated
on a per granularity basis. duplicated.

If the routing routing optimization requires the use of different BGP policy
policies for each element (e.g.
peer), the different sets of clients, a larger part of the decision
process needs to be duplicated, 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 Pahse Phase 1. This nedded is needed for use cases involved
involving traffic engineering perspective, or dedicating certain exit points for
certain clients points. clients. In the latter case, the user MAY specify and apply
a general policy on the route reflector to select for a subset set of exit points as the
candidate exit points for its clients. For a
given client, the
policy SHOULD also allow the operator to select different candidate
exit points for different address families. Regular path selection,
including client's perspective IGP based best path selection stated
above, will be applied to the candidate paths to select the final
paths to advertise to the clients.

4. Solution Interactions

4.1. IGP and policy based optimal route refresh

Depending on the actual deployment scenarios, service providers may
configure IGP based optimal route reflection or policy based optimal
route reflection. It is also possible to configure both approaches
together. In cases where both are configured together, policy based
optimal route reflection MUST be applied first to select the
candidate paths, then IGP based optimal route reflection can be
applied on top of the candidate paths to select the final path to
advertise to the client.

The expected use case for optimal route reflection is to avoid
reflecting all paths to the client because the client either: does
not support add-paths or does not have the capacity to process all set of
the paths. Typically the route reflector would just reflect a single
optimal route to the client. However, the solutions MUST NOT prevent
reflecting more than one optimal path to the client as path diversity
may be desirable for load balancing or fast restoration. In cases
where add-path and optimal route reflection are configured together,
the route reflector MUST reflect n optimal paths to a client, where n
is the add-path count.

4.2. Add-paths plus IGP and policy optimal route refresh

The most complicated scenario is where add-path is configured
together with both IGP based and policy based optimal route
reflection. In this scenario, clients, the policy based optimal route
reflection MUST be applied first SHOULD in that case allow the
operator to select the different candidate paths (from
add-path). Subsequently, exit points for different
address families. Regular path selection, including IGP based optimal route reflection will be
applied on top perspective
for a set of clients as per Section 3.1, is then applied to the
candidate paths to select the best n final paths to advertise to the client.

4.3.
clients.

4. Implementation considerations

4.1. Likely Deployments and need for backup

With IGP based optimal route reflection, even though the IGP location
could be specified on a per route reflector basis or per peer/update
group basis or per peer basis, in reality, it's most likely to be
specified per peer/update group basis. All clients with the same or
similar IGP location can be grouped into the same peer/update group.
An IGP location is then specified for the peer/update group. The
location is usually specified as the location of one of the clients
from the peer group or an ABR to the area where clients are located.
Also, one or more backup locations SHOULD be allowed to be specified
for redundancy. Implementations may wish to take advantage of peer
group mechanisms in order to provide for better scalability of
optimal route reflector client groups with similar properties.

5. CPU and Memory Scalability

For IGP based optimal route reflection, 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, current SPF tree
generation code is 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 a route reflector whenever a topology
change is detected. Advanced optimizations like partial and
incremental SPF may also be exploited. The number of SPTs computed It 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.

For policy based optimal route reflection, there will be some
overhead to apply the policy to select the candidate paths. This
overhead is comparable to existing BGP export policies and therefore
should be manageable.

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
is 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. This
will consume CPU and memory resources on all BGP speakers in the
network. Building this client perspective into the route reflectors
seems appropriate.

6. Advantages and Deployment Considerations

The solutions described provide a model for integrating the client
perspective into the best path computation for route reflectors.
More specifically, the choice of BGP path factors in either the IGP
cost between the client and the nexthop (rather than the IGP cost
from the route reflector to the nexthop) or other user configured
policies.

The achievement of optimal routing relies upon all route reflectors
learning all paths that are eligible for consideration. In order to
satisfy this requirement, path diversity enhancing mechanisms such as
BGP add-path [RFC7911] may need to be deployed between route
reflectors.

Implementations considered compliant with this document allow the
configuration of a logical location from which the best path will be
computed, on the basis of either a peer, a peer group, or an entire
routing instance.

These solutions can be deployed in traditional hop-by-hop forwarding
networks as well as in end-to-end tunneled environments. In 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, in turn leading to
intra-domain forwarding loops.

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 switching has been replaced by end-to-end MPLS or IP
encapsulation.

As per above, these approaches reduce the amount of state which needs
to be pushed to the edge of the network in order to perform hot
potato routing. The memory and CPU resources required at the edge of
the network to provide hot potato routing using these approaches is
lower than what would be required to achieve the same level of
optimality by pushing and retaining all available paths (potentially
10s) per each prefix at the edge.

The solutions above allow for a fast and safe transition to a BGP
control plane using 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 Best Path Selection from a different IGP best-path selection, location, it
should be self evident that this solution does not interfere with
policies enforced above IGP tie breaking tie-breaking in the BGP best path
algorithm.

7. Security Considerations

Similarly to [RFC4456], this extension to BGP does not change the
underlying security issues inherent in the existing IBGP [RFC4456].

It however enables the deployment of base BGP Route Reflection as
described in [RFC4456] to be possible using virtual compute
environments without any negative consequence on the BGP routing path
optimality.

This document does not introduce requirements for any new protection
measures, but it also does not relax best operational practices for
keeping the IGP network stable or to pace rate of policy based IGP
cost to next hops such that it does not have any substantial effect
on BGP path changes and their propagation to route reflection
clients.

8. IANA Considerations

This document does not request any IANA allocations.

9. 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 and Randy Bush for their
valuable input.

10. Contributors

Following persons substantially contributed to the current format of
the document:

Stephane Litkowski
Orange
9 rue du chene germain
Cesson Sevigne, 35512
France

stephane.litkowski@orange.com
Cisco System

slitkows.ietf@gmail.com

Adam Chappell
Interoute Communications
31st Floor
25 Canada Square
London, E14 5LQ
United Kingdom

adam.chappell@interoute.com
GTT Communications, Inc.
Aspira Business Centre
Bucharova 2928/14a
158 00 Prague 13 Stodulky
Czech Republic

adam.chappell@gtt.net

11. References

11.1. 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>.

[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.

[RFC5492] Scudder, J.

[RFC4456] Bates, T., Chen, E., and R. Chandra, "Capabilities Advertisement
with BGP-4", "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 5492, 4456, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>. 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.

[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>.

11.2. 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.

[RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities
Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
<https://www.rfc-editor.org/info/rfc1997>.

[RFC1998] Chen, E. and T. Bates, "An Application of the BGP
Community Attribute in Multi-home Routing", RFC 1998,
DOI 10.17487/RFC1998, August 1996,
<https://www.rfc-editor.org/info/rfc1998>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114,
RFC 4384, DOI 10.17487/RFC4384, February 2006,
<https://www.rfc-editor.org/info/rfc4384>.

[RFC4456] Bates, T., Chen, E.,

[RFC4364] Rosen, E. and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4456, 4364, DOI 10.17487/RFC4456, April 10.17487/RFC4364, February
2006,
<https://www.rfc-editor.org/info/rfc4456>.

[RFC4893] Vohra, Q. <https://www.rfc-editor.org/info/rfc4364>.

[RFC4798] De Clercq, J., Ooms, D., Prevost, S., and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, DOI 10.17487/RFC4893, May 2007,
<https://www.rfc-editor.org/info/rfc4893>.

[RFC5283] Decraene, B., F. Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", Faucheur,
"Connecting IPv6 Islands over IPv4 MPLS Using IPv6
Provider Edge Routers (6PE)", RFC 5283, 4798,
DOI 10.17487/RFC5283, July 2008,
<https://www.rfc-editor.org/info/rfc5283>. 10.17487/RFC4798, February 2007,
<https://www.rfc-editor.org/info/rfc4798>.

[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>.

[RFC5668] Rekhter, Y., Sangli, S., and D. Tappan, "4-Octet AS
Specific BGP Extended Community", RFC 5668,
DOI 10.17487/RFC5668, October 2009,
<https://www.rfc-editor.org/info/rfc5668>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.

[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
and K. Kumaki, "Distribution of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<https://www.rfc-editor.org/info/rfc6774>.

[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>.

[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>.

Appendix A. Appendix: alternative solutions with limited applicability

One possible valid solution or workaround to the best path selection
problem requires sending all domain external paths from the route
reflector to all its clients. This approach suffers the significant
drawback of pushing a large amount of BGP state and churn 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 [RFC7911] [RFC6774]
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.
When the route reflector chooses one set of n paths and distributes
them 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 exit 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.

Another possibility to optimize exit point selection is the
implementation of distributed route reflector functionality at key
IGP locations in order to ensure that these locations see their
viewpoints respected in exit selection. Typically, however, this
requires the installation of physical nodes to implement the
reflection, and if exit policy subsequently changes, the reflector
placement and position can become inappropriate.

To counter the burden of physical installation, it is possible to
build a logical overlay of tunnels with appropriate IGP metrics in
order to simulate closeness to key locations required to implement
exit policy. There is significant complexity overhead in this
approach, however, enough so to typically make it undesirable.

Trends in control plane decoupling are causing a shift from
traditional routers to compute virtualization platforms, or even
third-party cloud platforms. As a result, without this proposal,
operators are left with a difficult choice for the distribution and
reflection of address families with significant exit diversity:

o centralized path selection, and tolerate the associated suboptimal
paths, or

o defer selection to end clients, but lose potential route scale
capacity

The latter can be a viable option, but it is clearly a decision that
needs to be made on an application and address family basis, with
strong consideration for the number of available paths per prefix
(which may even vary per prefix range, depending on peering policy,
e.g. consider bilateral peerings versus onward transit arrangements)

Authors' Addresses

Robert Raszuk (editor)
Bloomberg LP
731 Lexington Ave
New York City, NY 10022
USA
NTT Network Innovations

Email: robert@raszuk.net

Christian Cassar
Tesla
43 Avro Way
Weybridge KT13 0XY
UK

Email: ccassar@tesla.com
Erik Aman
Telia Company
Solna SE-169 94
Sweden

Email: erik.aman@teliacompany.com erik.aman@aman.se

Bruno Decraene (editor)
Orange

Email: bruno.decraene@orange.com

Kevin Wang
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA

Email: kfwang@juniper.net