BGP Synchronization

Background Information

1. For a transit AS, where the iBGP routers have to transit non-BGP routers to get to another iBGP router, we can potentially blackhole routes if any of the internal non-BGP routers do not know the routes that the edge routers are advertising to other AS.
2. The way we can avoid blackholing routes is to not advertise the route to another AS if our IGP does not know it.  
3. The synchronization feature tells the BGP process to wait for the IGP to learn about the prefix before we can advertise it to another AS.

Summary

1. Older routers have synchronization enabled by default.  Newer routers have it disabled by default.
2. synchronization to tell the router to wait for IGP to learn the prefix before advertising it.
3. no synchronization to tell the router to not wait for IGP to learn the prefix.
4. If you are not running an IGP or the AS is not a transit AS, you should manually disable synchronization.
5. show ip protocols to see whether IGP Synchronization is enabled.

router bgp 1

  no synchronization

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html#synch