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Interactions Between BGP and IGPs
Synchronizing BGP with IGPs
Figure 36: Synchronization
Copyright © 2010, Juniper Networks, Inc.
Interactions between BGP and an interior gateway protocol are more likely to occur in
an enterprise topology than in a service provider topology. You can also encounter
interactions when configuring small test topologies. The main interaction factors are the
following:
Synchronization between BGP and IGPs
Administrative distances for routes learned from various sources
In Figure 36 on page 131, AS 100 provides transit service but does not run BGP on all of
the routers in the AS. In this situation, you must redistribute BGP into the IGP so that the
non-BGP routers—for example, router Albany—learn how to forward traffic to customer
prefixes. If BGP converges faster than the IGP, a prefix might be advertised to other ASs
before that prefix can be forwarded.
For example, suppose router LA advertises a route to router Boston using EBGP, and
router Boston propagates that route to router NY using IBGP. If router NY propagates
the route to router Chicago before the IGP within AS 100 has converged—that is, before
router Albany learns the route—then router Chicago might start sending traffic for that
route before router Albany can forward that traffic.
Synchronization solves this problem by preventing a BGP speaker from advertising a
route over an EBGP session until all routers within the speaker's AS have learned about
the route. If the AS contains routers connected by means of an IGP, the BGP speaker
cannot propagate a BGP route that it learned from a peer until an IGP route to the prefix
has been installed in the BGP speaker's IP routing table. The BGP speaker advertises the
BGP route externally even if the IGP route is better than the BGP route. By contrast, if
synchronization is disabled, a BGP speaker propagates a BGP route learned from a peer
only if it is the best route to the prefix in the IP routing table.
Synchronization is enabled by default. However, you must configure redistribution of
external routes into the IGP, or the routing tables will not receive the IGP routes.
Chapter 1: Configuring BGP Routing
131
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