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DebugPrivileged EXEC

debug ip bgp

Enables debugging of BGP events and updates to troubleshoot BGP neighbor relationships and route advertisement issues.

Syntax·Privileged EXEC
debug ip bgp

When to Use This Command

  • Diagnosing why a BGP session is flapping or not establishing
  • Verifying BGP route advertisements and withdrawals between peers
  • Troubleshooting BGP path selection or missing routes
  • Monitoring BGP keepalive and update messages in real time

Command Examples

Basic BGP debugging with neighbor IP

debug ip bgp 192.168.1.1 updates
BGP: 192.168.1.1 sending UPDATE about 10.0.0.0/24 (path: 65001) - NEXT_HOP 192.168.1.1
BGP: 192.168.1.1 received UPDATE about 172.16.0.0/16 (path: 65002) - NEXT_HOP 192.168.1.2
BGP: 192.168.1.1 received UPDATE about 10.0.0.0/24 - withdrawn
BGP: 192.168.1.1 sending KEEPALIVE
BGP: 192.168.1.1 received KEEPALIVE

Each line shows a BGP event. 'sending UPDATE' indicates the router is advertising a route; 'received UPDATE' means it learned a route from the neighbor. The path shows AS_PATH. 'withdrawn' means the route is being removed. KEEPALIVE messages maintain the session.

Debug BGP events for all neighbors

debug ip bgp events
BGP: 192.168.1.1 went from Active to OpenSent
BGP: 192.168.1.1 went from OpenSent to OpenConfirm
BGP: 192.168.1.1 went from OpenConfirm to Established
BGP: 192.168.1.1 went from Established to Idle (Keepalive timer expired)

Shows state transitions of BGP neighbor sessions. 'Established' is the desired state. Transitions to 'Idle' indicate a problem, such as a keepalive timeout or configuration mismatch.

Understanding the Output

The debug ip bgp command outputs real-time BGP events. For updates, each line shows the neighbor IP, direction (sending/received), prefix, AS_PATH, and NEXT_HOP. 'withdrawn' indicates route removal. For events, state transitions are shown: Idle, Active, OpenSent, OpenConfirm, Established. A healthy session stays in Established. Frequent transitions to Idle or Active indicate issues like misconfigured passwords, AS numbers, or network reachability. Keepalive messages should appear periodically; missing keepalives may cause session drops. Use 'undebug all' to stop debugging.

CCNA Exam Tips

1.

Remember that 'debug ip bgp' is CPU-intensive; always use specific keywords like 'updates' or 'events' to limit output.

2.

The CCNA exam may ask which debug command to use to see BGP route advertisements; answer: 'debug ip bgp updates'.

3.

Know that BGP states (Idle, Active, OpenSent, OpenConfirm, Established) are tested; Established is the goal.

4.

Be aware that 'debug ip bgp' requires privileged EXEC mode and can impact production routers.

Common Mistakes

Running 'debug ip bgp' without specifying a neighbor or filter, flooding the console with excessive output.

Forgetting to disable debugging with 'undebug all' or 'no debug ip bgp', causing performance degradation.

Misinterpreting 'withdrawn' as an error; it is normal when routes are removed due to policy or network changes.

Related Commands

show ip bgp

Displays the BGP routing table, showing learned BGP routes and their attributes, used for troubleshooting BGP path selection and verifying route advertisement.

show ip bgp neighbors

Displays detailed information about BGP neighbor sessions, including state, timers, and advertised/received prefixes, used to verify BGP peering and troubleshoot neighbor relationships.

undebug all

Disables all active debug operations on the router, used to stop debugging output and reduce CPU load.

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