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BGP and BGP4+ Introduction
Software Reference Supplement for SwitchBlade® x8112, x908, x900 and x610 Series Switches
C613-50032-01 REV D AlliedWare Plus
TM
Operating System - Software Version 5.4.3-2.6 1.11
The synchronization feature tells a BGP router not to consider an iBGP-learned route as
best unless the prefix is learned locally or via an IGP and is currently in the IP routing table.
BGP synchronization prevents reachability issues by preventing a BGP router advertising
routes learned from an iBGP peer unless the route is learned locally, or via an IGP.
If the route does not exist in the routing table, because an IGP has not learned the route,
then BGP will not use or advertise the learned prefix. If BGP advertised the route to an
external peer, then the external peer would consider that path valid. This is the resulting
problem if synchronization is not enabled and there are non-BGP routers in the transit AS.
The eBGP peer would send traffic to the BGP router advertising the prefix and that router
would be able to pass the traffic on since the IGP routers between the two iBGP routers do
not know the prefix.