wdiff draft-ietf-idr-bgp-optimal-route-reflection-20.txt draft-ietf-idr-bgp-optimal-route-reflection-21.txt

IDR Working Group R. Raszuk, Ed.
Internet-Draft Bloomberg LP
Intended status: Standards Track C. Cassar
Expires: July 11, December 18, 2020 Tesla
E. Aman
Telia Company
B. Decraene Decraene, Ed.
Orange
K. Wang
Juniper Networks
January 8,
June 16, 2020

BGP Optimal Route Reflection (BGP-ORR)
draft-ietf-idr-bgp-optimal-route-reflection-20
draft-ietf-idr-bgp-optimal-route-reflection-21

Abstract

This document proposes a solution for defines an extension to BGP route reflectors to allow
them reflectors. On route
reflectors, BGP route selection is modified in order to choose the
best path for their clients that the clients
themselves would have chosen under standpoint, rather than from the same conditions, without
requiring further state route
reflectors standpoint. Multiple type of granularity are proposed,
from a per client BGP route selection or any new features to be placed a per peer group,
depending on the
clients. 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 IGP location is suboptimal. This
facilitates, for example, best exit point policy (hot potato
routing). 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. Best path selection is
performed in each route reflector based on the IGP cost from a configured virtual
selected location in the link state IGP. The location can be the same for all clients or
different per peer/update group or per peer. Best path selection can
also be performed based on user configured policies in each route
reflector.

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
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 July 11, December 18, 2020.

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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

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

1. Definitions of Terms Used in This Memo

NLRI - Network Layer Reachability Information.

RIB - Routing Information Base.

AS - Autonomous System number.

VRF - Virtual Routing and Forwarding instance.

PE - Provider Edge router

RR - Route Reflector

POP - Point Of Presence

L3VPN - Layer 3 Virtual Private Networks [RFC4364]

6PE - IPv6 Provider Edge Router

IGP - Interior Gateway Protocol

SPT - Shortest Path Tree

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

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

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

best path selection - the decision process detailed in [RFC 4271]
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. Authors

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

adam.chappell@interoute.com

3. 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.

3.1. 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 best 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 for
the route reflector - not necessarily the one best for the route
reflector 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, 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, 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.

3.2. Existing/Alternative Solutions

One possible valid solution or workaround

3. Modifications to the best path BGP Best Path selection
problem requires sending all domain external paths from

The core of this solution is the ability for an operator to specify
on a per route reflector to all its clients. This approach suffers basis, or per peer/update group basis, or
per peer basis 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 IGP location of locations. the route reflector. This could easily result in tens of paths for
each prefix that would need core
ability enables the route reflector to be distributed send to clients.

Notwithstanding this drawback, there are a number given group of reasons for
sending more than just the single best path
clients routes with shortest distance to the clients. Improved
path diversity at next hops from the edge is a requirement
position of the selected IGP location. This core ability provides
for fast connectivity
restoration, freedom of route reflector physical location, and a requirement for effective BGP level load
balancing.

In practical terms, add/diverse path deployments [RFC7911] [RFC6774]
are expected allows
transient or permanent migration of this network control plane
function to result in the distribution an arbitrary location.

The choice of 2, 3, specific granularity (route reflector basis, peer/
update group basis, or n (where n peer peer basis) is
a small number) good paths rather than all domain external paths.
When configured by the route reflector chooses network
operator. An implementation is considered compliant with the
document if it supports at least one set listed grouping of IGP location.

For purposes 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 selection, the context perspective of the problem
described above, those n paths will not necessarily include the
closest exit point out a client can
differ from that of the network for each a route reflector
client. The mechanisms proposed or another client in this document are likely to be
complementary two
distinct ways:

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

it can have a different routing policy.

These factors correspond to mechanisms aimed at improving path diversity.

Another possibility the issues described earlier.

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

The first change is related to 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 cost 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 BGP Next Hop,
which is possible to
build a logical overlay of tunnels with appropriate IGP metrics done in
order to simulate closeness to key locations required to implement
exit policy. There is significant complexity overhead the step e) in this
approach, however, enough so to typically make it undesirable.

Trends the BGP decision process. The
change consists in control plane decoupling are causing a shift using the IGP cost 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 different source than
the associated suboptimal
paths, or

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

The latter can be a viable option, but it second change is clearly a decision that
needs to be made on an application and address family basis, with
strong consideration for the number granularity 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)

4. Two Proposed Solutions

This document is describes two solution the BGP decision process,
to allow a for running multiple decisions process using different
perspective or policies.

A route reflector to
choose the best path for its clients that the clients themselves
would have chosen had they considered the same set of candidate
paths.

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

it can have a different routing policy.

These factors correspond to the issues described earlier.
Accordingly, this document specifies two distinct modifications to
the best path algorithm, to address these two distinct factors. A
route reflector can implement either or both of the modifications in
order to allow it 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/
diverse paths 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.

4.1. Client's Perspective

3.1. IGP Based Best Path Selection

The core of this solution is the ability for an operator to specify
on a per route reflector basis, or per peer/update group basis, or
per peer basis the virtual IGP location placement of the route
reflector. This core ability enables the route reflector to send to
a given group of clients routes with shortest distance to the next
hops from the position of the configured virtual IGP location. This
core ability provides for freedom of route reflector 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, or peer peer basis) is left as an implementation
decision. An implementation is considered compliant with the
document if it supports at least one listed grouping of virtual IGP
location. a different SPT root

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 does not apply to path selection preference based on
other policy steps and provisions.

The computation of

In addition to the virtual IGP location with change specified in [RFC4456] section 9, the BGP
Decision Process Tie Breaking rules ([RFC4271] Sect. 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 above
described granularity 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
the selected IGP location.

This extension requires the knowledge of the IGP topology in order to
be able to compute the shortest path tree rooted on any location and
in particular on the selected IGP locations. 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 IGP location is outside of the scope of this
document. The operator may configure it manually, 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 BGP change (BGP or IGP protocol changes, IGP) on the
clients, as all required changes are contained within limited to the route reflector
implementation. reflector.

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

4.1.1.

3.1.1. Restriction when BGP next hop is BGP prefix

In situations where the BGP next hop is a BGP prefix 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.

4.2. Client's Perspective Policy Based

3.2. Best Path Selection

Optimal route reflection based on virtual IGP location could reflect Selections granularity

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

This requires running multiple best path based
on factors beyond IGP cost. Examples include, but not limited to:

o Selecting selections or multiple
subset of the best path for selection. If the clients from a 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. 9.1.2.2, needs to be duplicated
on a per granularity basis. If the routing routing optimization
requires the use of different BGP policy for each element (e.g.
peer), the 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 1. This nedded for use cases involved traffic
engineering
perspective.

o Dedicating perspective, or dedicating certain exit points for
certain ingress clients points.

The

In the latter case, the user MAY specify and apply a general policy
on the route reflector to select a subset 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.

Since the policy is applied on the route reflector on behalf of its
clients, the route reflector will be able to reflect only the optimal
paths to its clients. An additional advantage of this approach is
that configuration need only be done on a small number of route
reflectors, rather than on a significantly larger number of clients.

4.3.

4. Solution Interactions

4.3.1.

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 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.

4.3.2.

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, the policy based optimal route
reflection MUST be applied first to select the candidate paths (from
add-path). Subsequently, IGP based optimal route reflection will be
applied on top of the candidate paths to select the best n paths to
advertise to the client.

4.3.3.

4.3. Likely Deployments and need for backup

With IGP based optimal route reflection, even though the virtual 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 peer/update group. A virtual
An IGP location is then specified for the peer/
update peer/update group. The virtual
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 virtual 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
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.

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 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.

7. Security Considerations

This document does not introduce any new

Similarly to [RFC4456], this extension to BGP functionality therefor
it does not change the
underlying security issues inherent in the existing IBGP path propagation when BGP Route Reflection [RFC4456] is
used. [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
Adam Chappell
Interoute Communications
31st Floor
25 Canada Square
London, E14 5LQ
United Kingdom

adam.chappell@interoute.com

11. References

10.1.

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. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.

[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>.

10.2.

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>.

[RFC4384]

[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., 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>.

[RFC4893] Vohra, Q. 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., Le Roux, JL., and I. Minei, "LDP Extension Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.

[RFC4893] Vohra, Q. 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., Le Roux, JL., and I. Minei, "LDP Extension
for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
DOI 10.17487/RFC5283, July 2008,
<https://www.rfc-editor.org/info/rfc5283>.

[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 Inter-Area Label Switched Paths (LSPs)", RFC 5283,
DOI 10.17487/RFC5283, July 2008,
<https://www.rfc-editor.org/info/rfc5283>.

[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., the distribution and K. Kumaki, "Distribution
reflection of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<https://www.rfc-editor.org/info/rfc6774>.

[RFC7911] Walton, D., Retana, A., Chen, E., address families with significant exit diversity:

o centralized path selection, and J. Scudder,
"Advertisement 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 Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>. 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

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
Bruno Decraene (editor)
Orange
38-40 rue du General Leclerc
Issy les Moulineaux cedex 9 92794
France

Email: bruno.decraene@orange.com

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

Email: kfwang@juniper.net