Synchronization is a technique for automatically redistributing routes between the BGP and your IGPs. The main goal of BGP synchronization is to prevent routing inconsistencies. It serves to guarantee that only entries that exist in the IGP's IP routing table are included in the BGP routing tables. The BGP rule of synchronization states that if your autonomous system is passing traffic from another AS to a third AS, BGP should not advertise a route until the entire collection of local AS routers has learned about the route via an IGP routing protocol. BGP will wait until IGP has propagated the route within the AS before advertising it to external neighbor routers.
For example, suppose that a BGP router advertised a route to external network 10.10.10.0 without first allowing IGP to flood this route information throughout the local AS. If another BGP router received a packet destined for network 10.10.10.0 without receiving the update, this second router would discard the packet. BGP synchronization can be disabled, but it is only safe to do so when full mesh connectivity exists between all IBGP routers within the AS. After the entries between the tables are synchronized, routes can be redistributed between the protocols without the risk of black holes.