BGP troubleshooting basics: sessions, prefixes, path selection

A field guide for the four most common BGP problems — neighbor stuck in Active, missing prefixes, wrong best path, and MED/AS-path surprises.

BGP is deceptively simple: two routers exchange prefixes over TCP/179. When it goes wrong, however, the failure modes are almost always the same four.

1. Neighbor stuck in Active

show ip bgp summary
! State = Active means "trying to connect but failing"

Checklist:

Reachability — can you ping the neighbor's IP?
TCP 179 open on both sides (ACLs, firewalls).
Correct remote-as — an AS mismatch shows as Active too.
Update-source interface reachable from the peer.
eBGP multi-hop needed? neighbor X ebgp-multihop 2.

2. Session up but no prefixes received

show ip bgp neighbors X.X.X.X received-routes
show ip bgp neighbors X.X.X.X advertised-routes

Common causes:

The peer isn't advertising them — check on their side.
Prefix-list or route-map filtering inbound.
Max-prefix limit hit — %BGP-3-MAXPFX in logs.
Soft-reconfiguration required for received-routes — enable neighbor X soft-reconfiguration inbound.

3. Wrong best path

show ip bgp X.X.X.X/YY
show ip route X.X.X.X

Remember BGP path selection (simplified):

Highest weight (Cisco-local).
Highest local-preference.
Locally originated over learned.
Shortest AS-path.
Lowest origin (IGP < EGP < incomplete).
Lowest MED (same-AS peers only by default).
eBGP over iBGP.
Lowest IGP metric to next-hop.

4. Route flapping

show ip bgp dampening flap-statistics
show logging | i BGP

Flapping wastes CPU and destabilizes the RIB. Tools:

Route dampening (careful — deprecated by some standards bodies).
BFD for faster failure detection (down in ~150 ms instead of 90 s).
Investigate the interface — flapping is almost always a physical/L2 issue.

Sanity commands to memorize

show ip bgp summary
show ip bgp
show ip bgp neighbors X.X.X.X
show ip bgp regexp _65000_
clear ip bgp X soft in     ! non-disruptive refresh