With synchronization enabled, BGP will not advertise routes to an EBGP speaker unless that route is also known via the IGP protocol.
Synchronization
When an AS provides transit service to other ASs and if there are non-BGP routers in the AS, transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic via an IGP. The BGP synchronization rule 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. The topology shown in demonstrates the synchronization rule.
Figure 12-6 Synchronization
In , Router C sends updates about network 170.10.0.0 to Router A. Routers A and B are running IBGP, so Router B receives updates about network 170.10.0.0 via IBGP. If Router B wants to reach network 170.10.0.0, it sends traffic to Router E. If Router A does not redistribute network 170.10.0.0 into an IGP, Router E has no way of knowing that network 170.10.0.0 exists and will drop the packets.
If Router B advertises to AS 400 that it can reach 170.10.0.0 before Router E learns about the network via IGP, traffic coming from Router D to Router B with a destination of 170.10.0.0 will flow to Router E and be dropped.
This situation is handled by the synchronization rule of BGP, which states that if an AS (such as AS 100 in ) 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 170.10.0.0 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.
shows a topology in which it is desirable to disable synchronization.
Figure 12-7 Disabled Synchronization
The following commands configure Routers A, B, and C:
neighbor 3.3.3.4 remote-as 100
neighbor 2.2.2.1 remote-as 300
neighbor 1.1.1.2 remote-as 400
neighbor 3.3.3.3 remote-as 100
neighbor 1.1.1.1 remote-as 100
The no synchronization router configuration command causes Router B to put 170.10.0.0 in its IP routing table and advertise it to Router D without learning network 170.10.0.0 via an IGP.
Reference: Using the Border Gateway Protocol for Interdomain Routing http://www.cisco.com/en/US/docs/internetworking/case/studies/icsbgp4.html#wp19382
