Troubleshooting BGP Sessions: A Checklist

Session will not establish

1. Physical and VLAN. Confirm the interface is up and the VLAN ID matches on both sides.
2. IP reachability. Ping the neighbour address. If it fails, check addressing and subnet masks.
3. ASNs. Each side must configure the other’s ASN correctly.
4. Authentication. If MD5 or TCP-AO is used, keys must match exactly.
5. TCP port 179. Ensure no firewall or ACL blocks it.
6. Multihop. If peers are not directly connected, multihop must be configured.

Session is up but routes are missing

1. Outbound policy on the advertising side — is the prefix allowed?
2. Inbound policy on the receiving side — is it filtered?
3. Prefix existence — is the route actually in the advertising router’s table?
4. Next-hop reachability — routes with unreachable next hops are not used.
5. Max-prefix — has the limit been exceeded?

Session flapping

Look for physical errors, MTU mismatches, CPU load and timer settings. Enable BFD carefully — aggressive timers on an unstable link can make flapping worse.

Check from outside

For public routes, use our looking glass to confirm what the internet sees.