When configuring BGP, several common mistakes can cause session failures, missing routes, or improper route propagation. These mistakes can cause sessions to remain in Idle/Active states or prevent advertised routes from being installed in the routing table.
Below are the most frequent errors, their causes, and how to troubleshoot them effectively. To view a detailed explanation for each issue, simply click the provided links.
| Issue | Symptom | Solution |
|---|---|---|
| AS Number Mismatch | BGP session remains in Idle/Active | Ensure the remote-as values match the neighbor’s AS. |
| Missing network Statement | Locally originated prefixes aren’t advertised | Add network <subnet> mask <mask> under BGP. |
| No IP Connectivity | BGP session never transitions from Idle | Verify IP addressing and connectivity (use ping, no shutdown, same subnet). |
| Loopback Peering Without ebgp-multihop | BGP peering remains Idle when using loopbacks | Use directly connected interfaces or configure ebgp-multihop (and update-source if needed). |
| Incorrect Static Routes for Next-Hop | Routes appear in show ip bgp but not in show ip route | Add or correct static routes to ensure the next-hop is reachable. |
| Synchronization Not Disabled | BGP routes not advertised to eBGP peers | Disable synchronization with no synchronization. |
| Auto-Summary Not Disabled | Routes are summarized to classful boundaries | Disable auto-summary with no auto-summary. |
| Incorrect Next-Hop Handling (Next-Hop Unreachable) | Routes are learned but not installed due to unreachable next-hop | Either add a static route for the next-hop or configure next-hop-self to rewrite it. |
show ip bgp summary
show ip bgp
show ip route
ping <neighbor-IP>
clear ip bgp * <-- reset the BGP establishments if needed