BGP: Configuring Route Maps
Clauses
There are two types of clauses that can be present in a route map entry:
match clauses, which specify attributes or prefixes to match on
set clauses, which specify the changes to be made to attribute values
A given route map entry can never have more than one match clause, but it can have multiple set clauses.
Configuring a match clause
When you configure a match clause, you can match on one of the attributes listed in the following sections.
An AS path list
For information about creating an AS path list, see "AS path lists" on page 17.
Once you have made the path list, you can apply it to the match clause of a route map entry by using the commands:
awplus(config)#
This clause will cause the router to go through the entries in the AS path list. If one of the entries in the AS path list matches the AS path in the update packet, and the action on that AS path list entry is permit, then the packet is deemed to have matched this route map match clause. The fact that there are two levels of matching going on, and effectively two different uses of the permit/deny parameters in the one clause, can be a bit confusing.
When working out the correct way to configure a clause that matches on AS path list, it is important to first think about how you are configuring the match criteria within the AS path list, and then separately think about the actions of the route map entry.
In particular, it is important to note that in this context, the parameters in the AS path list do not indicate whether the matching update packet (remember that the AS Path attribute is only set once in an update packet) is being allowed or dropped; "permit" and "deny" simply indicate whether the update packet is deemed to match or not match the AS path list. This is different to the case where the AS path list itself is being used as a filter.
Just as an example, consider these configurations:
Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes Page 27