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IP Routing

Troubleshooting BGP

Cisco - Troubleshooting BGP

Document ID: 22166

Updated: Feb 03, 2012

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Introduction

This document provides troubleshooting information for common problems with Border Gateway Protocol (BGP). To see more information, or to go to the next flowchart, click the command boxes in red.

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 Output Interpreter (registered customers only) to display potential issues and fixes. To use Output Interpreter (registered customers only) , you must be a registered customer, be logged in, and have JavaScript enabled.

Before You Begin

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites

There are no specific prerequisites for this document.

Components Used

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

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

Main Troubleshooting Flowchart

bgp_trouble_main.jpg

Troubleshooting BGP Neighbor Establishment

bgp_trouble_neighbor.jpg

Note: *Sample log messages to be checked when neighbor is not coming 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

Note: **Example of ping with packet size and enable does not fragment bit in 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: **Also, check the duplex settings of the interface.

Note: If the reset has occurred due to interface flapping, disable the eBGP's fast failover by issuing 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.

Troubleshooting Routes Missing from the Routing Table

bgp_trouble_route_missing.jpg

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 should be advertised.

Troubleshooting Multihoming Inbound

bgp_trouble_multi_in.jpg

Troubleshooting BGP Route Advertisement

bgp_trouble_route_adv.jpg

Troubleshooting Multihoming Outbound

bgp_trouble_multi_out.jpg

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

Updated: Feb 03, 2012
Document ID: 22166