BGP Synchronization

BGP is an exterior gateway protocol, which means it does not keep track of routing within it's own AS. For BGP to work properly, you must be running an interior gateway protocol such as RIP, IGRP, OSPF, IS-IS or EIGRP.

A Cisco router running BGP will not advertise a route unless the route appears in the IP routing table and there is a network statement authorizing BGP to advertise the route.

While a network administrator can configure a network statement at any time, this will not guarantee the advertisement of the route. A route to an interior host will not appear in the IP table until it has been selected by an interior protocol as the best route and inserted in the IP routing table. Once the internal route appears, BGP and the interior routing protocol are said to be 'synchronized'.

This is necessary to prevent routing loops. A BGP speaking router will only advertise it's own routes, and will not announce any of it's iBGP neighbor's routes to any other iBGP peer. BGP does this to prevent routing loops within the AS.