In this figure, routers A, B, C, and D are all running IBGP and an IGP with each other. There are no matching IGP routes for the BGP routes (routers A and B are not redistributing the BGP routes into the IGP). Routers A, B, C, and D have IGP routes to the internal networks of AS 65500 but do not have routes to external networks such as 172.16.0.0.
Router B advertises the route to 172.16.0.0 to the other routers in AS 65500 using IBGP. If synchronization is on, routers A, C, and D do not use the route to 172.16.0.0, nor does router A advertise that route to router E in AS 64520. Router B uses the route to 172.16.0.0 and installs it in its routing table. If router E receives traffic that is destined for network 172.16.0.0, it does not have a route for that network and cannot forward the traffic.
If synchronization is off (the default) in AS 65500, routers A, C, and D can use the route to 172.16.0.0 and install the route in their routing tables even if there are no matching IGP routes for the BGP routes (assuming that routers A, C, and D can reach the next-hop address for 172.16.0.0). Router A advertises the route to router E. Router E then has a route to 172.16.0.0 and may send traffic that is destined for that network. Router E sends the packets to router A, and router A forwards them to router C. Router C learns a route to 172.16.0.0 via IBGP; therefore, router C forwards the packets to router D. Router D forwards the packets to router B. Router B forwards the packets to router F for network 172.16.0.0.
In modern autonomous systems, because the size of the Internet routing table is large, redistributing from BGP into an IGP is not scalable; therefore, most modern autonomous systems run full-mesh IBGP and do not require synchronization. Advanced BGP configuration methods, for example, using route reflectors and confederations, reduce the full-mesh requirements.
© 2006 Cisco Systems, Inc.
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