bgp as-path-list

BGP enabled devices use routing updates to exchange network routing information with each other. This information includes route details, such as the network number, path specific attributes, and the list of ASNs (Autonomous System Numbers) that a route traverses to reach a destination. This list is contained in the AS path.

An AS path ACL (access control list) filters AS paths (routes) included in routing updates. Each AS path access list consists of deny and/or permit rules that define regular expressions (match criteria). When configured and applied on inbound and outbound routing updates, the BGP AS path attributes are matched against the regular expressions specified in the AS path ACL. In case of a match, the route is filtered and an action (deny or permit) is applied. Once a match is made subsequent rules in the AS path access list are not processed.

AS path access lists also help prevent looping within an AS. Routing loops are prevented by rejecting routing updates containing local ASNs. Since local ASNs indicate that the route has already traveled through that autonomous system, by rejecting them looping is avoided.

To navigate to the AS path configuration instance, use the following command:

<DEVICE>(config)#bgp as-path <AS-PATH-LIST-NAME>

<DEVICE>(config-bgp-as-path-list-<AS-PATH-LIST-NAME>)#?
BGP AS Path List Mode commands:
  deny     Specify packets to reject
  no       Negate a command or set its defaults
  permit   Specify packets to forward

  clrscr   Clears the display screen
  commit   Commit all changes made in this session
  do       Run commands from Exec mode
  end      End current mode and change to EXEC mode
  exit     End current mode and down to previous mode
  help     Description of the interactive help system
  revert   Revert changes
  service  Service Commands
  show     Show running system information
  write    Write running configuration to memory or terminal

<DEVICE>(config-bgp-as-path-list-<AS-PATH-LIST-NAME>)#