AS Path List

About this task

BGP uses a routing algorithm to exchange network reachability information with other BGP supported devices. Network availability ad reachability information is exchanged between BGP peers in routing updates. This information contains a network number, path specific attributes and the list of autonomous system numbers a route transits to reach a destination. This list is contained in the AS path. BGP prevents routing loops by rejecting any routing update that contains a local autonomous system number, as this indicates the route has already traveled through that autonomous system and a loop would be created. BGP‘s routing algorithm is a combination of a distance vector routing algorithm and AS path loop detection.

The AS path contains a set of numbers for passing routing information. A BGP supported device adds its own autonomous system number to the list when it forwards an update message to external peers.

To define an AS path list:

Procedure

Select the Configuration > Network > BGP > AS Path List.
Select Add to create a new AS path list or Edit to modify the attributes of a selected path list. Existing policies can be copied or renamed as needed.

Set the following AS Path List settings:


Regular Expression	Provide a 64 character maximum regular expression unique to the AS path list rule. Regular expressions are used to specify patterns to match community attributes.
Allow	Use the drop-down menu to Permit or Deny requests for network access using the defined AS path list. The default setting is deny.

Click OK to save the changes, Reset to revert to the last saved configuration or Exit to close the screen.