Allied Telesis x908, X8100 manual Another example, An outgoing filter that uses an AS-path list

BGP: Configuring AS Path Filters

9.Check that the AS path list shows the two filter entries:

awplus(config-router)#do show ip as-path-access-list

AS path access list list1 deny 23456

permit 34568

An outgoing filter that uses an AS-path list

Another example

1.Create an AS-PATH list that denies empty AS Paths, but allows AS Paths that contain the AS number 34567.

ip as-path access-list example deny ^$

ip as-path access-list example permit 34567

2.Apply this as the out route map for neighbor 45.45.45.46. neighbor 45.45.45.46 filter-list example out

3.Shut down the neighbor, and then bring it up again.

awplus(config-router)#neighbor 45.45.45.46 shutdown awplus(config-router)#neighbor 45.45.45.46 no shutdown

4.There are no routes advertised to the other side, so that says that the AS Path in the update packets is not 34567 at the point where the updates are passed to the out-going filter.

5.Now remove the deny entry for empty AS-paths, and add an allow entry for empty AS_paths.

(config)#no ip as-path access-list example deny ^$ (config)#ip as-path access-list example permit ^$

The as-path list becomes:

ip as-path access-list example permit 34567 ip as-path access-list example permit ^$

6.Shut down the neighbor, and then bring it up again.

awplus(config-router)#neighbor 45.45.45.46 shutdown awplus(config-router)#neighbor 45.45.45.46 no shutdown

Now the routes are received by the neighbor. That does demonstrate that the AS-paths in the updates are empty at the moment they are examined by the filter. But, in the neighbor's

Page 20 Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes