Network Working Group P. Marques
Internet-Draft
Intended status: Standards Track R. Fernando
Expires: September 10, 2011 E. Chen
P. Mohapatra
Cisco Systems
March 9, 2011
Advertisement of the best external route in BGP
draft-ietf-idr-best-external-03.txt
Abstract
The base BGP specifications prevent a BGP speaker from advertising
any route that is not the best route for a BGP destination. This
document specifies a modification of this rule. Routes are divided
into two categories, "external" and "internal". A specification is
provided for choosing a "best external route" (for a particular value
of the Network Layer Reachability Information). A BGP speaker is
then allowed to advertise its "best external route" to its internal
BGP peers, even if that is not the best route for the destination.
The document explains why advertising the best external route can
improve convergence time without causing routing loops. Additional
benefits include reduction of inter-domain churn and avoidance of
permanent route oscillation. The document also generalizes the
notions of "internal" and "external" so that they can be applied to
Route Reflector Clusters and Autonomous System Confederations.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on September 10, 2011.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Algorithm for selection of best external route . . . . . . . . 5
3. Advertisement Rules . . . . . . . . . . . . . . . . . . . . . 6
4. Consistency between routing and forwarding . . . . . . . . . . 6
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Fast Connectivity Restoration . . . . . . . . . . . . . . 8
5.2. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . 9
5.3. Reducing Persistent IBGP oscillation . . . . . . . . . . . 9
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The base BGP specifications prevent a BGP speaker from advertising
any route that is not the best route for a BGP destination. This
document specifies a modification of this rule. Routes are divided
into two categories, "external" and "internal". A specification is
provided for choosing a "best external route" (for a particular value
of the Network Layer Reachability Information). A BGP speaker is
then allowed to advertise its "best external route" to its internal
BGP peers, even if that is not the best route for the destination.
The document explains why advertising the best external route can
improve convergence time without causing routing loops. Additional
benefits include reduction of inter-domain churn and avoidance of
permanent route oscillation.
The document also generalizes the notions of "internal" and
"external" so that they can be applied to Route Reflector Clusters
[RFC4456] and Autonomous System Confederations [RFC5065]. More
specifically, two routers in the same route reflector cluster having
an IBGP session between them are defined to be "internal" peers,
whereas two routers in different clusters having an IBGP session are
defined to be "external" peers. Similarly, two routers in the same
member AS of a confederation having an IBGP session between them are
"internal" peers, whereas two routers in different member ASs of a
confederation having a confed EBGP session between them are defined
to be "external" peers. The definition of "best external route"
ensues from this definition in that it is the most preferred route
among those received from the "external" neighbors.
Advertising the best external route, when different from the best
route, presents additional information into an IBGP mesh which may be
of value for several purposes including:
o Faster restoration of connectivity, by providing additional paths,
that may be used to fail over in case the primary path becomes
invalid or is withdrawn.
o Reducing inter-domain churn and traffic blackholing due to the
readily available alternate path.
o Reducing the potential for situations of permanent IBGP route
oscillation, as discussed in some scenarios [RFC3345].
o Improving selection of lower MED routes from the same neighboring
AS.
This document defines procedures to select the best external route
for each destination. It also describes how above benefits are
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realized with best external route announcement with the help of
certain scenarios.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Algorithm for selection of best external route
Given that the intent in advertising an external route, when the best
route for the same destination is an internal route, is to provide
additional information into the IBGP mesh into which a route is
participating, it is desirable to take into account the routes
received from internal neighbors in the selection process.
We propose a route selection algorithm that selects a total order
between routes and which selects the same best route as the one
currently specified [RFC4271].
In order to achieve this, we need to introduce the concept of route
group. For a given NLRI, suppose the BGP decision process has run
through all the steps prior to the MED comparison step (as defined in
section 9.1.2.2 of [RFC4271]. Look at the set of routes that are
still under consideration at that time. Now partition this set into
a number of disjoint route groups, where two routes are in the same
group if and only if the neighbor AS of each route is the same.
Routes are ordered within a group via MED or subsequent route
selection rules.
The order of all routes for the same destination is determined by the
order of the best route in each group.
As an example, the following set of received routes:
Path AS MED rtr_id
a 1 10 10
b 2 5 1
c 1 5 5
d 2 20 20
e 2 30 30
f 3 10 20
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Figure 1: Path Attribute Table
Would yield the following order (from the most to the least
preferred):
b < d < e < c < a < f
In this example, comparison of the best route within each group
provides the sequence (b < c < f). The remaining routes are ordered
in relation to their respective group best.
The first route in the above ordering is indeed the best route for a
given destination. Eliminating the best route and executing the
above steps leads us to a new total order of the routes. The route
to be advertised to a particular domain is selected by choosing the
most preferred route that is external to that particular domain in
the above order. Note that whenever the overall best route is
external it will automatically be selected by this algorithm.
3. Advertisement Rules
1. In an AS domain, if a router has installed an internal route as
best, it should advertise its "best external route" (as defined
in the draft) to its internal neighbors.
2. In a Cluster domain, if a router (route reflector) has installed
an external route as best, it should advertise its "best internal
route" to its external neighbors. (Advertising to internal
neighbors is unchanged.) Similarly, if the route reflector has
installed an internal route as best, it should advertise its
"best external route" to its internal (client) peers. In order
for the reflector to be able to advertise the best external route
into the cluster, it is necessary that client-to-client
reflection be disabled, since its advertisement may otherwise
contain the best route within the cluster domain.
3. In a Confederation Member domain, if a router (confederation
border router) has installed an internal route as best, it
advertises its best external route to its internal neighbors.
However, if it has installed an external route as best, it
advertises its best internal route to its external neighbors.
4. Consistency between routing and forwarding
The BGP protocol, as defined in [RFC1771], specifies that a BGP
speaker shall advertise to its internal peers the route with the
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highest degree of preference among routes to the same destination
received from external neighbors.
This section discusses problems present with the approach described
in [RFC1771] and the next section offers an alternative algorithm to
select a best external route which can be advertised to an IBGP mesh.
The internal update advertisement rules contained in the original
BGP-4 specification [RFC1771] can lead to situations where traffic is
forwarded through a route other than the route advertised by BGP.
Inconsistencies between forwarding and routing are highly
undesirable. Service providers use BGP with the dual objective of
learning reachability information and expressing policy over network
resources. The latter assumes that forwarding follows routing
information.
Consider the Autonomous system presented in figure 1, where r1 ... r4
are members of a single IBGP mesh and routes a, b, and c are received
from external peers.
AS 1 (c)
|
+----+ +----+
| r1 |...........| r2 |
+----+ +----+
.
.
.
.
.
.
+----+ +----+
| r3 |...........| r4 | --- ebgp --- AS X
+----+ +----+
/ \
/ \
AS 1 (a) AS 2 (b)
Figure 2: Inconsistency in Routing
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Path AS MED rtr_id
a 1 10 1
b 2 5 10
c 1 5 5
Figure 3: Path Attribute Table - 2
Following the rules as specified in [RFC1771], router r3 will select
path (b) received from AS 2 as its overall best to install in the
Loc-Rib, since path (b) is preferable to path (c), the lowest MED
route from AS 1. However for the purposes of Internal Update route
selection, it will ignore the presence of path (c), and elect (a) as
its advertisement, via the router-id tie-breaking rule.
In this scenario, router r4 will receive (c) from r1 and (a) from r3.
It will pick the lowest MED route (c) and advertise it out via ebgp
to AS X. However at this point routing is inconsistent with
forwarding as traffic received from AS X will be forwarded towards AS
2, while the ebgp advertisement is being made for an AS 1 path.
Routing policies are typically specified in terms of neighboring
ASes. In the situation above, assuming that AS 1 is network for
which this AS provides transit services while AS 2 and AS X are peer
networks, one can easily see how the inconsistency between routing
and forwarding would lead to transit being inadvertently provided
between AS X and AS 2. This could lead to persistent forwarding
loops.
Inconsistency between routing and forwarding may happen, whenever a
bgp speaker chooses to advertise an external route into IBGP that is
different from the overall best route and its overall best is
external.
5. Applications
5.1. Fast Connectivity Restoration
When two exits are available to reach a particular destination and
one is preferred over the other, the availability of an alternate
path provides fast connectivity restoration when the primary path
fails.
Restoration can be quick since the alternate path is already at hand.
The border router could precompute the backup route and preinstall it
in FIB ready to be switched when the primary goes away. Note that
this requires the border router that's the backup to also preinstall
the secondary path and switch to it on failure.
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5.2. Inter-Domain Churn Reduction
Within an AS, the non availability of backup best leads to a border
router sending a withdraw upstream when the primary fails. This
leads to inter-domain churn and packet loss for the time the network
takes to converge to the alternate path. Having the alternate path
will reduces the churn and eliminates packet loss.
5.3. Reducing Persistent IBGP oscillation
Advertising the best external route, according to the algorithm
described in this document will reduce the possibility of route
oscillation by introducing additional information into the IBGP
system.
For a permanent oscillation condition to occur, it is necessary that
a circular dependency between paths occurs such that the selection of
a new best path by a router, in response to a received IBGP
advertisement, causes the withdrawal of information that another
router depends on in order to generate the original event.
In vanilla BGP, when only the best overall route is advertised, as in
most implementations, oscillation can occur whenever there are 2 or
clusters/sub-ASes such that at least one cluster has more than one
path that can potentially contribute to the dependency.
6. Deployment Considerations
The mechanism specified in the draft allows a BGP speaker to
advertise a route that is not the best route used for forwarding.
This is a departure from the current behavior. However, consistency
in the path selection process across the AS is still guaranteed since
the ingress routers will not choose the best-external route as the
best route for a destination in steady state (for the same reason
that the BGP speaker announcing the best-external route chose an IBGP
route as best instead of the externally learnt route). Though it is
possible to alter this assurance by defining route policies on IBGP
sessions, use of such policies in IBGP is not recommended, especially
with best-external announcement turned on in the network. It is also
worth noting that such inconsistency in routing and forwarding is
mitigated in a tunneled network.
7. Acknowledgments
This document greatly benefits from the comments of Yakov Rekhter,
John Scudder, Eric Rosen, Jenny Yuan, and Jay Borkenhagen.
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8. IANA Considerations
This document has no actions for IANA.
9. Security Considerations
There are no additional security risks introduced by this design.
10. Normative References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, August 2002.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007.
Authors' Addresses
Pedro Marques
Email: pedro.r.marques@gmail.com
Rex Fernando
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: rex@cisco.com
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Enke Chen
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: enkechen@cisco.com
Pradosh Mohapatra
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: pmohapat@cisco.com
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