Troubleshoot Common BGP Issues

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Updated:August 26, 2025
Document ID:22166

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Introduction

  

This document describes how to troubleshoot common issues with Border Gateway Protocol (BGP). 

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Background Information

   

This document provides flowcharts for several options to troubleshoot BGP issues. 

  

If you have the output of a show ip bgp, show ip bgp neighbors, show ip bgp summary, or show tech-support command from your Cisco device, you can use Cisco CLI Analyzer to display potential issues and fixes. To use Cisco CLI Analyzer, you must be a registered Cisco user.

Note: Only registered Cisco users can access internal tools and information.

Main Troubleshoot Flowchart

  

BGP Main FlowchartBGP Main Flowchart

Troubleshoot BGP Neighbor Establishment

  

BGP Neighbor

  

  This is a sample log message that must be checked when the neighbor does not come up:

BGP_SESSION-5-ADJCHANGE: neighbor[ip address] IPv4 Unicast topology base removed
  from session Peer closed the session
BGP_SESSION-5-ADJCHANGE: neighbor[ip address] IPv4 Unicast topology base removed
  from session Unknown path error

  

  This is an example of ping with packet size, and enable does not contain fragment bit in the IP header:

Router#ping 10.10.10.2 size 1400 df-bit

Type escape sequence to abort.
Sending 5, 1400-byte ICMP Echos to 10.10.10.2, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/37/84 ms

  

Note: You need to check the duplex settings of the interface.

  

Note: If the reset has occurred due to interface flapping, disable the eBGP fast failover with the no bgp fast-external-fallover command. By default, BGP resets the neighborship if the link used in order to reach the neighbor goes down. Disable this feature under BGP Configuration in order to keep BGP stable and prevent the interface from flapping.

  

If flapping occurs due to high CPU, refer to Troubleshooting High CPU Utilization on Cisco Routers.

Troubleshoot Routes Missing from the Routing Table

  

BGP Router Missing

Note: If the BGP routes are not in the routing table, verify if the network statement under the BGP configuration is correct.

Note: In the debug ip bgp x.x.x.x updates command, x.x.x.x is the neighbor to which the route must be advertised.

Troubleshoot Multihoming Inbound

BGP Multi-in

Troubleshoot BGP Route Advertisement

BGP Route Advanced

Troubleshoot Multihoming Outbound

BGP Multi-out

Related Information

BGP Neighbor Establishment Routes not in Routing Table BGP Route Advertisement Multihoming/Loadsharing BGP Error Message BGP Memory Consumption BGP across PIX Firewall Troubleshooting Flapping BGP Routes Configuring and Verifying the BGP Conditional Advertisement Feature show ip bgp neighbor ip mtu show ip bgp neighbor Why Do BGP Neighbors Toggle Between Idle, Connect, and Active States? Using the Extended ping and Extended traceroute Commands Configuring the BGP Maximum-Prefix Feature Why Do BGP Neighbors Toggle Between Idle, Connect, and Active States? Open a TAC case Neighbor Establishment Troubleshooting Flapping BGP Routes (Recursive Routing Failure) Route Advertisement Using the BGP for Interdomain Routing Open a TAC case Sample Configuration for Load Sharing Multihoming Outbound Load Sharing Using the Loopback Address as a BGP Neighbor Load Sharing when Dualhomed to One ISP through Multiple Local Routers Using BGP for Interdomain Routing Routes Announced Using a Basic Network Statement Routes Announced Using the aggregate-address Command Routes Announced Using the Network Statement with a Mask Routes Announced Using a Basic Network Statement Unable to Announce iBGP-Learned Routes Contact the TAC Load Sharing Using the Loopback Address as a BGP Neighbor Using BGP for Interdomain Routing Load Sharing when Multihomed to Two ISPs Through a Single Local Router Load Sharing when Multihomed to Two ISPs via Multiple Local Routers Troubleshooting Load Balancing Over Parallel Links Using CEF Sample Configuration for BGP with Two Different Service Providers (Multihoming) How to Use HSRP to Provide Redundancy in a Multihomed BGP Network

Revision History

Revision Publish Date Comments
4.0
26-Aug-2025
Recertification
3.0
25-Oct-2022
Update and Recertification
1.0
31-Mar-2002
Initial Release
TAC Authored

Contributed by Cisco Engineers

  • Cisco TAC Engineers

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