BGP: Configuring Route Maps
BGP: Configuring Route Maps
Route maps are very powerful and flexible entities. Therefore, the configuring of route maps must, by necessity, be relatively complex. The purpose of this section of the document is to understand route maps piece by piece and thereby build up a full understanding of how all the parts fit together.
Structure of a route map
There are various levels of structure within a route map:
A route map is an entity with a name
Each route map consists of multiple entries, identified by sequence numbers
Each entry can consist of multiple clauses
In effect, an entry defines an individual filter. It can have a match clause that defines what it will match on, and it can have multiple set clauses that can specify actions to be taken. An update packet is matched against each entry in turn. Once an entry is found that matches the packet, the action(s) associated with that entry is (are) performed, and no further entries are considered.
For example, if you create an entry that will permit an update packet, followed by an entry that would deny that packet, the packet is permitted. As another example, if you create two conflicting set clauses, in different entries, the first change is applied, not the second.
A
<set clause(s)>
There is no need for a command that creates a
The sequence numbers determine the order in which the entries are applied to update packets. A
There is an implicit match all filter at the end of the route map. The action on that implicit entry is deny. By default, any update packet that does not explicitly match any particular entry in the route map will be dropped.
You can change this by ending the route map with a “permit all” clause, such as the following:
awplus(config)#
Page 26 Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes
