BGP synchronization rule refers to prefix synchronization between BGP and IGP. If it is enabled, BGP speakers will not advertise routes learned from an iBGP peer unless the destination described in the route is also reachable through the local IGP.

Below is the topology used for simulating BGP Synchronization rule by enabling command “SYNCHRONIZATION” under BGP process.

R7-R4-R5 and R6 is running OSPF as routing protocol. R6 and R7 is having iBGP peering with source address loopback 0. Loopback 100 is advertised on routers R6 and R7 and synchronization is enabled under BGP process.

R7 is learning R6 loopback 100 but that prefix is not considered as best prefix because it is not available in the routing table. (Read More About Different Types of BGP Tables)

Now check the output of routing table:-

This means, If a BGP learned prefix is not synchronized, the prefix will not be inserted into the routing table and will not be advertised to external peers.