BGP and IGP synchronization
Enable BGP and IGP route synchronization in an AS to avoid giving wrong directions to routers.
If a non-BGP router works in an AS, it can discard a packet because a destination is unreachable. As shown in Figure 81, Router E has learned a route of 8.0.0.0/8 from Router D via BGP. Router E then sends a packet to 8.0.0.0/8 through Router D, which finds from its routing table that Router B is the next hop (configured using the peer next-hop-local command). Because Router D has learned the route to Router B via IGP, it forwards the packet to Router C through route recursion. Router C is unaware of the route 8.0.0.0/8, so it discards the packet.
Figure 81: BGP and IGP synchronization in an AS
For this example, if synchronization is enabled, and the route 8.0.0.0/24 received from Router B is available in its IGP routing table, Router D adds the route into its BGP routing table and advertises the route to the EBGP peer.
You can disable the synchronization feature in the following situations:
The local AS is not a transitive AS (AS20 is a transitive AS in the above figure).
Routers in the local AS are IBGP fully meshed.