show and debug Commands for BGP-Related Troubleshooting

show and debug Commands for BGP- Related Troubleshooting

Cisco IOS Software offers descriptive show commands and debugs to aid in trouble-shooting BGP-related problems. Furthermore, most of the debugs can be run with access lists to limit the output displayed because excessive debug output can severely degrade router performance. Some of the most commonly used show and debug commands in troubleshooting BGP problems in Cisco routers are as follows :

• show ip bgp prefix

• show ip bgp summary

• show ip bgp neighbor [ address ]

• show ip bgp neighbors [ address ] [ advertised-routes ]

• show ip bgp neighbors [ routes ]

• debug ip bgp update [ access-list ]

• debug ip bgp neighbor-ip-address updates [ access-list ]

show ip bgp prefix Command

This is probably the most widely used BGP show command to check the BGP path entry for prefix in BGP table. Among other things, the output shows all BGP attributes assigned to the prefix and all available paths from multiple neighbors.

show ip bgp summary Command

This command gives a summarized list of the status of all BGP neighbors, the number of prefixes received from each peer, and local BGP parameters.

show ip bgp neighbor [ address ] Command

This command displays details about the BGP neighbor, including its status, the number of updates sent and received, and TCP statistics.

show ip bgp neighbors [ address ] [ advertised-routes ] Command

This command displays routes advertised to neighbors and is used in troubleshooting cases when neighbors don't receive some or all BGP routes.

show ip bgp neighbors [ routes ] Command

This command displays routes received from neighbors and is used in troubleshooting cases when the local routers don't receive some or all BGP routes.

debug ip bgp update [ access-list ] Command

This is the most commonly used BGP debug to troubleshoot problems in BGP path advertisement. The access-list option limits the output display; otherwise , if the number of prefixes is huge, this output can severely degrade router performance and also can reload the router in worst cases. Both standard and extended access lists can be used.

Standard Access List Usage

  debug ip bgp update 1   access-list 1 permit host 100.100.100.0  

With standard access lists, the host 100.100.100.0 means that if a BGP update contains 100.100.100.0, only the debug displays the output. Unlike extended access lists, standard access lists do not give any option to limit the output based on the mask of the prefix.

Extended Access List Usage

  debug ip bgp update 101   access-list 101 permit ip host 100.100.100.0 host 255.255.255.0  

For the preceding extended access list, only BGP updates related to 100.100.100.0/24 dis-play. The first portion of the access list, host 100.100.100.0, means that the prefix to display output is 100.100.100.0. The second portion, host 255.255.255.0, requires the mask of 100.100.100.0 to be Class C 255.255.255.0 (/24).

debug ip bgp neighbor-ip-address updates [ access-list ] Command

This debug command is similar to the previous one, except that it gives the option of displaying debug output on a per-neighbor basis.