Checking AS Path Length

You can use the bgp maxas-limit command to prevent the forwarding of routes having AS paths longer than a specified limit.

bgp maxas-limit

• Use to require BGP to check the AS path in all received update messages.
• If a received AS path is longer than the specified limit:
  • The route is stored in the BGP routing table and therefore is displayed by the show ip bgp commands.
  • The route is not a candidate for being selected as a best path, is not stored in the forwarding information base, and is not propagated to external or internal peers.
• Changes in the limit do not affect routes previously received. Clearing the BGP sessions (clear ip bgp) forces a resend of all routes; the new limits are then applied on receipt of the routes.
• Example

  host1(config-router)#bgp maxas-limit 42

• Causes BGP to check the AS path of all routes received after you issue the command.

  To apply the new behavior to routes that are already present in the BGP routing table, you must use the clear ip bgp command to perform a soft clear or hard clear of the current BGP session.

• Use the no version to halt checking of received AS path lengths.
• See bgp maxas-limit

If you use the fields as-path option with the show ip bgp command, the display indicates routes whose AS path exceeds the limit. The following display illustrates the result of setting the AS path length limit to 5:

host1:3# show ip bgp fields intro best peer loc-pref as-path
Local router ID 13.13.13.3, local AS 200
 10 paths, 5 distinct prefixes (520 bytes used)
 6 paths selected for route table installation
 14 path attribute entries (1943 bytes used)
Status codes: > best
 Prefix              Peer            LocPrf AS-path
  10.23.40.1/32      192.168.13.1    200    100 211 32 15 67 44 (too long)
> 10.23.40.1/32      172.123.23.2    100    100 211
> 10.23.40.2/32      192.168.13.1    200    100 211 32 15 67
  10.23.40.2/32      172.123.23.2    100    100 211 32
> 10.23.40.3/32      192.168.13.1           100 211 32 15
  10.23.40.3/32      172.123.23.2           100 211 32 15
  10.23.40.4/32      192.168.13.1    100    100 211 32
> 10.23.40.4/32      172.123.23.2    200    100 211 32 15 67
> 10.23.40.5/32      192.168.13.1    100    100 211
  10.23.40.5/32      172.123.23.2    200    100 211 32 15 67 44 (too long) 

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