Specifying Policy with Aggregate Routes

You can associate a routing policy when configuring an aggregate route’s destination prefix in the routes part of the aggregate statement. Doing so provides the equivalent of an import routing policy filter for the destination prefix. That is, each potential contributor to an aggregate route, along with any aggregate options, is passed through the policy filter. The policy then can accept or reject the route as a contributor to the aggregate route and, if the contributor is accepted, the policy can modify the default preferences.

The following algorithm is used to compare two aggregate contributing routes in order to determine which one is the primary or preferred contributor:

1. Compare the protocol’s preferences of the contributing routes. The lower the preference, the better the route. This is similar to the comparison that is done while determining the best route for the routing table.
2. Compare the protocol’s preferences2 of the contributing routes. The lower preference2 value is better. If only one route has preferences2, then this route is preferred.
3. The preference values are the same. Proceed with a numerical comparison of the prefix values.
   1. The primary contributor is the numerically smallest prefix value.
   2. If the two prefixes are numerically equal, the primary contributor is the route that has the smallest prefix length value.
4. At this point, the two routes are the same. The primary contributor does not change. An additional next hop will be available for the existing primary contributor.

A rejected contributor still can contribute to a less specific aggregate route. If you do not specify a policy filter, all candidate routes contribute to an aggregate route.

To associate a routing policy with an aggregate route, include the policy statement when configuring the route:

For a list of hierarchy levels at which you can configure this statement, see the statement summary section for this statement.

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