IP Routing

Troubleshooting BGP

You might encounter your first problem with BGP when configuring a new peering relationship. The BGP session between the peers may not enter the Established state (the output of show ip bgp neighbor may show other states, such as Idle, Connect, or Active). Here are the first steps you should take when troubleshooting BGP:

1. Check the infrastructure between the peers: an ICMP ping test between the peers is a quick test of layer-3 reachability between the peers.

2. If layer 3 reachability exists between the peers, the configuration of the peers may be in error. Check the BGP configuration of neighbor IP addresses and AS numbers on each peer.

3. Check for IP filters (access lists) or firewalls between the peers that would prevent a BGP TCP session (on port 179) from being formed.

4. Is there a BGP version mismatch between the peers? Look for the BGP neighbor ip-address version number sub-command.

Once the neighbors have entered into the Established state, you may find that no prefixes or only some prefixes are being exchanged. Here are the steps you should take:

• If no prefixes are being exchanged, you may want to go back and make sure that the peers are indeed in an Established state.

• If some but not all prefixes are being exchanged, check to see if a filter (an access list, a prefix filter, or an attribute filter such as an AS-PATH filter) is blocking the update. The show ip bgp neighbor ip-address command will show the filters that are applied to the BGP session.

• Filter changes ...