The BGP Synchronization rule states that – “A BGP router should not use or advertise a route learned by iBGP to an external neighbor, unless that route is local or learned from the IGP (EIGRP, OSPF, RIP or ISIS). Because this synchronization rule is a loop prevention mechanism in old days and it is enabled by default, since not all routers were BGP speakers then. Hence synchronization is enabled to ensure that there are no black holes inside the ISP AS. Intermediate routers only taking part in IGP would know how to route traffic in the destination.
But now this rule comes disabled by default and one could also disable it if one of the following conditions is true:
- Your AS does not pass traffic from one AS to another AS
- All the transit routes in your AS run BGP.
When you disable synchronization it allows routers to carry fewer routes in the IGP and allow BGP to converge more quickly.