Synchronization Rule: "Do not advertise a route if your IGP does not have it in its routing table."
© 2002, Cisco Systems, Inc. All rights reserved. Cisco CCIE Prep v1.0—Module 8-26
The BGP rule of synchronization states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. In other words, it states "Do not advertise a route if the IGP does not have it in its routing table."
This is an example of the rule of synchronization and how it works. In the diagram, Router C sends updates about network 220.127.116.11 to Router A. Routers A and B are running iBGP, so Router B receives updates about network 18.104.22.168 via iBGP. If Router B wants to reach network 22.214.171.124, it sends traffic to Router E. If Router A does not redistribute network 126.96.36.199 into an IGP, Router E has no way of knowing that network 188.8.131.52 exists and will drop the packets.
If Router B advertises to AS 400 that it can reach 184.108.40.206 before Router E learns about the network via IGP, traffic coming from Router D to Router B with a destination of 220.127.116.11 will flow to Router E and be dropped.
"To characterize the set of policy decisions that can be enforced using BGP, one must focus on the rule that a BGP speaker advertise to its peers (other BGP speakers which it communicates with) in neighboring ASs only those routes that it itself uses."
This situation is handled by the synchronization rule of BGP, which states that if an AS (such as AS 100 in the diagram) passes traffic from one AS to another AS, BGP does not advertise a route before all routers within the AS (in this case, AS 100) have learned about the route via an IGP. In this case, Router B waits to hear about network 18.104.22.168 via an IGP before it sends an update to Router D. In some cases, you might want to disable synchronization. Disabling synchronization allows BGP to converge more quickly, but it might result in dropped transit packets.
You can disable synchronization if one of the following conditions is true: ■ Your AS does not pass traffic from one AS to another AS.
■ All the transit routers in your AS run BGP.
For example, to turn off synchronization on Router A, issue the commands:
RouterA(config)router bgp 100 RouterA(config-router)no synchronization
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