BGP Troubleshooting Scenarios

Here’s a set of BGP troubleshooting scenarios with descriptions and associated diagrams for better understanding:

BGP Troubleshooting Scenario 1: Neighbor Not Establishing (Idle State)

Problem

A BGP neighbor remains in the Idle state, indicating no TCP session establishment.

Cause

• TCP port 179 blocked by a firewall.
• Incorrect neighbor IP address or AS number.

Solution

1. Verify reachability using ping or telnet [neighbor-IP] 179.
2. Check the BGP configuration on both routers.
3. Ensure firewalls or ACLs are not blocking port 179.

Related: Common TCP FIN Issues and How to Troubleshoot Them

BGP Troubleshooting Scenario 2: Missing Prefixes in Routing Table

Problem

Certain prefixes are missing from the BGP table on a router.

Cause

• Filters (prefix-lists, route-maps) are blocking routes.
• The advertised route is not in the neighbor’s routing table.

Solution

1. Check the neighbor’s advertised prefixes:

show ip bgp neighbor [neighbor-IP] advertised-routes

2. Verify filters with:

show run | include prefix-list

3. Ensure the route exists in the source router’s routing table.

BGP Troubleshooting Scenario 3: Flapping BGP Routes

Problem

A prefix keeps appearing and disappearing in the BGP routing table, causing instability.

Cause

• Unstable link between peers.
• Fluctuating IGP route (OSPF/EIGRP) being redistributed into BGP.

Solution

1. Stabilize the link.
2. Use BGP route dampening:

bgp dampening

3. Adjust IGP timers to reduce route changes.

BGP Troubleshooting Scenario 4: High CPU Usage on Router

Problem

A router experiences high CPU usage due to excessive BGP updates or large routing tables.

Cause

• Multiple BGP peers with large numbers of prefixes.
• Inefficient filters or route summarization.

Solution

1. Use summarization to reduce the number of advertised routes:

aggregate-address [summary-prefix] [mask]

2. Apply filters to limit received prefixes:

ip prefix-list limit-permissions

3. Optimize BGP timers if updates are frequent.

BGP Troubleshooting Scenario 5: AS Path Loop Prevention

Problem

A router rejects a route due to AS path loop detection.

Cause

The same AS appears in the AS path due to improper configuration or route reflection.

Solution

1. Use the allowas-in command if the loop is intentional and safe.
2. Recheck route-reflector configurations.

ABOUT THE AUTHOR

Rashmi Bhardwaj

I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”

I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.

I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)