Applying Filters to Received Routes

By applying filters of routing policies to routing protocols, you can filter the received routes.

Usage Scenario

When exchanging routes on a network, devices need to accept only required routes. In this situation, you can define a filter (such as an IP prefix list, ACL, or a route-policy) for a routing policy, apply the filter to the routing protocol, and run the filter-policy command specified with the filter in the related protocol view to filter received routes. Then, devices accept only the routes that meet the matching rules.

The function of the filter-policy import command varies the protocol type. And the functions to a distance-vector protocol and a link-state protocol are as follows:

Distance-vector protocol

A distance-vector protocol generates routes based on the routing table. Therefore, the command filters the routes received from neighbors and those to be advertised to neighbors.
Link-state protocol

A link-state protocol generates routes based on the link state database (LSDB). The filter-policy command does not affect any Link State Advertisement (LSA) or LSDB.

After routes are received, the filter-policy command determines which routes to be added from the protocol routing table to the local core routing table. Therefore, this command takes effect on the local core routing table rather than the protocol routing table.

BGP has the powerful filtering function. For the configuration of BGP routing policies, refer to "BGP Configuration."
For details of the filter-policy and import-route commands and their applications in RIP, OSPF, IS-IS, and BGP, refer to related configurations.

Pre-configuration Tasks

Before applying filters to received routes, complete the following tasks:

Configuring RIP to Filter the Received Routes: You can configure an inbound or outbound filtering policy by specifying Access Control Lists (ACLs) and IP address prefix lists to filter routes to be received and advertised. You can also configure a device to receive only the RIP packets from a specified neighbor.

Configuring OSPF to Filter the Received Routes: After a filtering policy is configured for the OSPF routes that need to be delivered to the routing management module, only the routes that match the policy will be added to the routing table.

Configuring IS-IS to Filter the Received Routes: By configuring IS-IS to filter the received routes, you can control the number of IS-IS routes to be added to the IP routing table, and thus reduce the size of the IP routing table.

Verifying the Configuration of Applying Filters to Received Routes: After applying filters to the received routes, verify information about the routing table of each protocol.