In Figure 36, 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.
Figure 36: Synchronization
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.
 |
Note: When you create an address family for a VRF, synchronization is automatically disabled for that address family. |
If synchronization is enabled and if redistribution is configured for the networks in Figure 36, router NY checks its IGP routing table for a route to 192.56.0.0/16 when it learns about the prefix from the IBGP session with router Boston. If the route is not present, the prefix is not reachable through router Albany, so router NY does not advertise it as available. Router NY keeps checking its IGP routing table; if the route appears, router NY knows that it can pass traffic to the prefix and advertises the route by means of EBGP to router Chicago.
In practice, service providers rarely redistribute BGP into an IGP because existing IGPs cannot handle the full Internet routing table (about 100,00 routes). Instead, all routers in an AS typically run BGP; in these cases it is advisable to turn synchronization off everywhere.
 | Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |