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.

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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.

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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.

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3. See All BGP Routes Received

Command:

show ip bgp neighbors <neighbor-IP> received-routes

Requires soft-reconfiguration inbound to 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.

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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

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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.

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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

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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.

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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.

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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.

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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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