Configuring BGP4

12.Select the Save link at the bottom of the dialog. Select Yes when prompted to save the configuration change to the startup-config file on the device’s flash memory.

Defining Route Maps

A route map is a named set of match conditions and parameter settings that the router can use to modify route attributes and to control redistribution of the routes into other protocols. A route map consists of a sequence of up to 50 instances. If you think of a route map as a table, an instance is a row in that table. The router evaluates a route according to a route map’s instances in ascending numerical order. The route is first compared against instance 1, then against instance 2, and so on. As soon as a match is found, the router stops evaluating the route against the route map instances.

Route maps can contain match statements and set statements. Each route map contains a “permit” or “deny” action for routes that match the match statements.

•If the route map contains a permit action, a route that matches a match statement is permitted; otherwise, the route is denied.

•If the route map contains a deny action, a route that matches a match statement is denied.

•If a route does not match any match statements in the route map, the route is denied. This is the default action. To change the default action, configure the last match statement in the last instance of the route map to “permit any any”.

•If there is no match statement, the software considers the route to be a match.

•For route maps that contain address filters, AS-path filters, or community filters, if the action specified by a filter conflicts with the action specified by the route map, the route map’s action takes precedence over the individual filter’s action.

If the route map contains set statements, routes that are permitted by the route map’s match statements are modified according to the set statements.

Match statements compare the route against one or more of the following:

•The route’s BGP4 MED (metric)

•A sequence of AS-path filters

•A sequence of community filters

•A sequence of address filters

•The IP address of the next hop router

•The route’s tag

•For OSPF routes only, the route’s type (internal, external type-1, or external type-2)

For routes that match all of the match statements, the route map’s set statements can perform one or more of the following modifications to the route’s attributes:

•Prepend AS numbers to the front of the route’s AS-path. By adding AS numbers to the AS-path, you can cause the route to be less preferred when compared to other routes on the basis of the length of the AS-path.

•Add a user-defined tag to the route or add an automatically calculated tag to the route.

•Set the community value.

•Set the local preference.

•Set the MED (metric).

•Set the IP address of the next hop router.

•Set the origin to IGP or INCOMPLETE.

•Set the weight.

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