Filter
Top Products
AS path access list list1
deny 23456
permit 34568
BGP: Confi
guring AS Path Filters
Page 20 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
Another example
An outgoing
filter that uses
an AS-path list
ip as-path access-list example deny ^$
ip as-path access-list example permit 34567
neighbor 45.45.45.46 filter-list example out
awplus(config-router)# neighbor 45.45.45.46 shutdown
awplus(config-router)# neighbor 45.45.45.46 no shutdown
(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 ^$
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
9. Check that the AS path list shows the two filter entries:
awplus(config-router)#
do show ip as-path-access-list
1. Create an AS-PATH list that denies empty AS Paths, but allows AS Paths that
contain the AS number 34567.
2. Apply this as the out route map for neighbor 45.45.45.46.
3. Shut down the neighbor, and then bring it up again.
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.
6. Shut down the neighbor, and then bring it up again.