BGP Synchronization

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.

BGP synchronization helps to prevent the phenomenon of a non-existent route known as a black hole . A black hole occurs when a destination is advertised to an EBGP peer before all the internal AS routers can reach that destination.

Current Cisco IOS releases all have synchronization enabled by default, although in future releases it will probably be disabled because most ISPs are running BGP on all their routers. Disabling synchronization can reduce convergence time between routers because fewer routes need to be advertised by each protocol. However, it should be disabled only if all routers in the BGP transit path run BGP or if no traffic moves through the AS to another AS (in other words, the AS is not serving as a transit AS). In these cases, disabling synchronization reduces the number of routes that must be stored by IGP and increases IBGP convergence time. Because of the cost and complexity of maintaining direct connections for all BGP routers in an AS ”a requirement of IBGP peer interactions ”most BGP ASs use an IGP for intra-AS communications. Usually, only ISP- related transit ASs run BGP on all routers in the transit path because the only purpose of the AS is to route traffic between other connected ASs from a fully meshed topology.