BGP Synchronization Rule
The BGP synchronization rule requires that when a BGP router receives
information about a network from an IBGP neighbor, it does not use that
information until a matching route is learned via an IGP or static route. It
also does not advertise that route to an EBGP neighbor unless a matching
route is in the routing table. In Figure 6-3, if RtrB advertises a route to RtrC,
then RtrC does not submit it to the routing table or advertise it to RtrD
unless it also learns the route from some other IGP source.
Recent IOS versions have synchronization disabled by default. It is usually safe
to turn off synchronization when all routers in the autonomous system run
BGP. To turn it off in earlier IOS versions, use the command no synchronization
under BGP router configuration mode.