BGP Troubleshooting Cheat Sheet

Aug 5, 2026

When a BGP session won’t come up, I work through the state machine top-down: Idle → Connect → Active → OpenSent → OpenConfirm → Established. Where it gets stuck tells you where to look.

1. Is the session even up?

show bgp ipv4 unicast summary
show ip bgp neighbors 10.0.0.1

If it’s stuck in Active, TCP/179 isn’t completing — check reachability, ACLs and ttl-security. If it flaps between OpenSent and Idle, suspect an ASN or router-id mismatch.

2. Are we exchanging routes?

show ip bgp neighbors 10.0.0.1 advertised-routes
show ip bgp neighbors 10.0.0.1 received-routes

No routes received? Check the neighbor’s outbound policy. No routes installed? Look at next-hop reachability and any inbound route-maps.

3. Common gotchas

• Next-hop not reachable → use next-hop-self on iBGP.
• Missing network statement or redistribution.
• Prefix-list / route-map silently dropping everything.
• MTU mismatch breaking large updates (ping with DF bit to confirm).

Keep this list handy — 90% of BGP issues fall into one of these buckets.