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"no synchronization" in Cisco BGP configurations

This text used to be on the home page, but Cisco now includes "no syncronization" in the default configuration, so it's unlikely anyone is still going to run into trouble because of this, so I've moved this to a separate page out of the way.

When you run BGP on two or more routers, you need to configure internal BGP (iBGP) between all of them. If those routers are Cisco routers, they won't work very well unless you configure them with no synchronization.

The no synchronization configuration command tells the routers that you don't want them to "synchronize" iBGP and the internal routing protocol such as OSPF. The idea behind synchronizing is that when you have two iBGP speaking routers with another router in between that doesn't speak BGP, the non-BGP router in the middle needs to have the same routing information as the BGP routers, or there could be routing loops. The way to make sure that the non-BGP router is aware of the routing information in BGP, is to redistribute the BGP routing information into the internal routing protocol.

By default, Cisco routers expect you to do this, and wait for the BGP routing information to show up in an internal routing protocol before they'll use any routes learned through iBGP. However, these days redistributing full BGP routing into another protocol isn't really done any more, because it's easier to simply run BGP on any routers in the middle.

But if you don't redistribute BGP into internal routing, the router will still wait for the BGP routes to show up in an internal routing protocol, which will never happen, so the iBGP routes are never used.

The "no synchronization" configuration command tells the routers they shouldn't wait for this synchronization, but just go ahead and use the iBGP routes.