Cisco BGP Troubleshooting: Common Issues and Fixes

BGP sessions that will not establish, prefixes that disappear, and neighbors that flap are the most common routing problems in service provider and enterprise networks. This Cisco-authored document condenses years of TAC experience into a practical troubleshooting flow: start with show ip bgp all summary, verify connectivity and configuration, then work through hold-timer, AFI/SAFI, next-hop, RIB and performance issues one by one. The commands and corrective actions below apply to Cisco IOS and IOS XE.

BGP Adjacency Down: First Checks

If a session is down, issue show ip bgp all summary to see the current state — IDLE or ACTIVE means the Finite State Machine has not reached Established. Then check:

• No connectivity: verify with ping (loopback-to-loopback when peering over loopbacks), check show ip route peer_IP, layer 1 state, and any firewall or ACL blocking TCP 179.
• Wrong AS: the log shows %BGP-3-NOTIFICATION: sent to neighbor ... 2/2 (peer in wrong AS) — correct the AS numbers.
• Duplicate router ID: %BGP-3-NOTIFICATION ... 2/3 (BGP identifier wrong) — set unique router IDs manually with bgp router-id X.X.X.X.
• Missing update-source: iBGP over loopbacks requires neighbor ip-address update-source interface-id.

Adjacency Bounces: Interface Flap and Hold Timer

If the neighbor continuously bounces, check for physical interface flaps with show interface and show logging, then verify that the hold timer (default 180s) is not expiring because of high CPU or packet loss. Debug with debug ip bgp and debug ip tcp transactions to see where the TCP session is being reset.

AFI/SAFI, Next-Hop and RIB Issues

Prefixes received but not installed usually indicate a next-hop reachability problem — the route's next hop must exist in the RIB or the BGP route stays hidden. Check show ip bgp for routes in the table, show ip bgp rib-failure for routes rejected by the RIB, and the best-path selection with show ip bgp bestpath.

High CPU and Slow Peer Handling

BGP scanner, router, I/O, open and event processes all consume CPU. Excessive packets in the BGP queue point at a slow peer — the slow-peer feature (e.g. neighbor X slow-peer detection threshold 120) removes the slow peer from the update path until it catches up. Memory issues are checked with show process memory and show ip bgp summary memory counters.

Also see our guides on BGP security with ASPA and BGP TCP-AO configuration on Arista EOS for additional BGP hardening and troubleshooting practice.

原文链接：https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/218027-troubleshoot-border-gateway-protocol-bas.html