Network Working Group P. Marques
Internet-Draft R. Fernando
Intended status: Standards Track Juniper Networks
Expires: January 28, 2009 E. Chen
P. Mohapatra
Cisco Systems
July 27, 2008
Advertisement of the best-external route to IBGP
draft-marques-idr-best-external-00.txt
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Abstract
This document makes a case and provides the rules for a border router
to advertise its best external route towards its IBGP peers when its
overall best is a route received from an IBGP peer.
The best external route may be different from the overall best route
installed in the Loc-Rib. Advertising the best-external route (when
different from the overall best route) into an IBGP helps in speeding
up routing convergence, has positive effects in reducing inter-domain
churn and in some limited scenarios could help avoid permanent IBGP
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route oscillation.
The document also extends this mechanism to route reflectors and
confederation border routers to advertise a best route that is
external to the cluster/domain.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Consistency between routing and forwarding . . . . . . . . . . 3
3. Algorithm for selection of best-external route . . . . . . . . 5
4. Route Reflection . . . . . . . . . . . . . . . . . . . . . . . 6
5. Confederations . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Fast Connectivity Restoration . . . . . . . . . . . . . . 7
6.2. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . 7
6.3. Reducing Persistent IBGP oscillation . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
The term best-external route describes the most preferred route among
the routes received by a router from its EBGP peers. The best-
external route might differ from the overall route installed in the
Loc-RIB in the case when the overall best route happens to be an
internal route. Advertising the best-external route, when different
from the overall best, 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.
In current networks, BGP is typically deployed in topologies that
include the use of route reflectors [RFC4456] and/or confederations
[RFC5065]. It is straightforward to extend the concept of "external"
route to a cluster or confed sub-AS. A route is considered
"external" if it has not been received from the cluster/sub-AS which
is being considered for advertisement.
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. 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
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.
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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 1: Inconsistency in Routing
Path AS MED rtr_id
a 1 10 1
b 2 5 10
c 1 5 5
Figure 2: Path Attribute Table
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
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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.
3. Algorithm for selection of best-external route
Given that the intent in advertising an external route, when the
overall best 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 interior neighbors in the selection process.
We propose a route selection algorithm that selects a global order
between routes and which selects the same overall best route as the
one currently specified [RFC4271].
In order to achieve this we need to introduce the concept of route
group. A route group is a set of routes to the same destination
received from the same neighboring AS and which is equal in terms of
route selection prior to the MED comparison step.
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.
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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
Figure 3: Path Attribute Table - 2
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 route to be advertised to the IBGP mesh or a given cluster/sub-AS
is selected by choosing the most preferred route that is external to
that particular domain. Note that whenever the overall best route is
external it will automatically be selected by this algorithm.
4. Route Reflection
A route reflector that chooses to implement this algorithm, will
advertise to its non-client IBGP peers, the most preferred path
received from its clients. This is referred to as the best intra-
cluster route. It will advertise to its client peers the most
preferred path received from a neighbor outside the cluster. This is
referred to as the best inter-cluster route.
In order for a reflector to be able to advertise the best of its
inter-cluster routes into a cluster it is necessary that client-to-
client reflection be disabled, since its advertisement may otherwise
5. Confederations
When a BGP speaker is configured as a confederation border router, it
shall consider the best-external route as follows:
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o When advertising into its sub-AS, it should select the most
preferred route not received from within its sub-AS.
o o When advertising into confed ebgp, it should select the most
preferred route not received from the neighboring sub-AS.
6. Applications
6.1. 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.
6.2. 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.
6.3. 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.
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7. Acknowledgments
This document greatly benefits from the comments of Yakov Rekhter,
John Scudder and Jenny Yuan.
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.
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Authors' Addresses
Pedro Marques
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Phone:
Email: roque@juniper.net
Rex Fernando
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Phone:
Email: rex@juniper.net
Enke Chen
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Phone:
Email: enkechen@cisco.com
Pradosh Mohapatra
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Phone:
Email: pmohapat@cisco.com
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