BGP notes

I'm currently setting up a site to advertise a new address block into our MPLS network.  We are using neighbor <peer IP> soft-reconfiguration inbound on all of our edge devices.

It is worth knowing that in order to update your BGP routing table, you must perform a 'clear ip bgp *' (which tears down ALL of your BGP neighbor relationships) or 'clear ip bgp <peer IP address>.'

This can have negative consequences, being that when you perform this command it will tear down the BGP neighbor relationship and cease communication until the neighbor relationship is rebuilt.  Unlike other interior routing protocols, which automatically update the neighbors with the newly advertised network, BGP peers with external groups..typically ISPs.

ISPs won't typically send their updated BGP routing table to you unless you force them.  The alternative to 'clear ip bgp*' is to perform a soft clear which was implemented in IOS 12.2.

When you perform the "neighbor A.B.C.D soft-reconfiguration inbound" router command - it shuts down the connection (just like a hard reset). When the neighbor resends you it's routing table, BGP stores a copy of the received routing table in memory. This 2nd copy takes up memory, but it serves a useful purpose. Now, when you apply policy changes to a neighbor or out a particular interface, you can configure "clear ip bgp * (or A.B.C.D) soft in" and run the saved copy of the neighbor's routes through your new bgp policy filters. You can make policy changes all you want and your neighbor never has to resend you its routing table, and you never have to tear down the connection between you and disrupt network communication.

The obvious drawback of the soft-reconfiguration inbound command is this does take up a lot of memory because the router stores all of the updates that it receives from the neighbor. So make sure the router has enough memory before turning it on.

It is worth noting that Cisco IOS automatically soft reconfiguration for outbound updates, but the above implementation is required for inbound updates.