Cisco BGP Troubleshooting: Essential Commands Every Network Engineer Should Know
When BGP (Border Gateway Protocol) misbehaves, the ripple effect can be massive—routes disappear, prefixes flap, and connectivity grinds to a halt. Whether you’re managing a large enterprise edge or an ISP core, knowing how to quickly diagnose BGP issues on Cisco routers is critical.
Here’s a hands-on guide to the most useful Cisco IOS BGP troubleshooting commands, broken down by scenario.
1. Check BGP Neighbor Relationships
The first step in BGP troubleshooting is verifying if neighbor sessions are up.
🔹 Command:
show ip bgp summary
🔍 What it tells you:
- Neighbor IPs
- BGP state (Idle, Active, Established, etc.)
- Number of prefixes received
- Uptime of the session
✅ Example Output:
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
192.0.2.1 4 64512 12345 12340 100 0 0 3d10h 101
If the State is not “Established”, you’ve got a problem.
2. Dig into BGP Neighbor Status
🔹 Command:
show ip bgp neighbors [<neighbor-IP>]
🔍 What it tells you:
- Session state and uptime
- Error messages and reset reasons
- Prefixes received and advertised
- BGP timers
- Capabilities like route refresh or 4-byte ASN support
🔧 Use Case:
Check for version mismatches, timer mismatches, or authentication failures.
3. See All BGP Routes Received
🔹 Command:
show ip bgp neighbors <neighbor-IP> received-routes
🛑 Requires
soft-reconfiguration inboundto be configured.
✅ Alternative (if soft-reconfig not enabled):
debug ip bgp updates
OR use:
show ip bgp
and look for routes tagged with the correct next-hop and origin ASN.
4. View the BGP RIB (Routing Information Base)
🔹 Command:
show ip bgp
🔍 What it tells you:
- All BGP prefixes in the RIB
- Best path indicator (
*>) - AS path, next-hop, metric
✅ Example:
*> 203.0.113.0/24 192.0.2.1 0 64512 i
Use this to verify if a route is:
- Being received
- Preferred
- Originating from the right peer
5. Check the BGP Table for a Specific Prefix
🔹 Command:
bashCopyEditshow ip bgp <prefix>
✅ Example:
bashCopyEditshow ip bgp 198.51.100.0/24
This pinpoints what path(s) are available and which one was selected as best.
6. Check Routes Advertised to a Peer
🔹 Command:
show ip bgp neighbors <neighbor-IP> advertised-routes
Use this to confirm your router is advertising the expected routes. If it’s not, verify:
- The route exists in the routing table
- It’s being redistributed into BGP
- Outbound route-maps aren’t filtering it
7. Debug Live BGP Updates
🔹 Command:
debug ip bgp
Or more targeted:
debug ip bgp updates
⚠️ Caution: Only use debug in controlled environments. It can overload the CPU on production routers.
8. Check BGP Flap Statistics
🔹 Command:
show ip bgp flap-statistics
🔍 What it tells you:
- Routes that are flapping
- Number of flaps and suppression status
- When the route was last suppressed
This is useful for diagnosing route dampening issues.
9. Clear or Reset BGP Sessions
🔹 Clear a specific neighbor (soft):
clear ip bgp <neighbor-IP> soft in
🔹 Hard reset (drops and restarts session):
clear ip bgp <neighbor-IP>
Use soft resets to refresh routes without impacting the session.
🔚 Conclusion
BGP is powerful—but also sensitive to misconfigurations and external factors. Mastering the Cisco IOS commands above will give you the confidence to diagnose BGP issues quickly and accurately.
Here’s a quick reference cheat sheet:
| Scenario | Command |
|---|---|
| BGP session down | show ip bgp summary |
| Detailed neighbor info | show ip bgp neighbors |
| Prefix not appearing | show ip bgp <prefix> |
| Not receiving prefixes | show ip bgp neighbors <ip> received-routes |
| Not advertising | show ip bgp neighbors <ip> advertised-routes |
| Flapping route | show ip bgp flap-statistics |
If your BGP session is in Active or Idle, start with checking:
- Peer reachability (ping/traceroute)
- AS number matches
- Authentication (if used)
- TTL (for eBGP multihop scenarios)
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