OSPF Route Filtering

Route filtering allows traffic flows to be manipulated at the area border router by filtering advertised routes to other routers.

Filter by Summarisation

The most simplistic method to filter a route for advertisement is to use the not-advertise option when configuring prefix summarisation. Using the not-advertise keyword prevents any type 3 LSAs being generated for any networks within that summarisation range.

This allows the routes to only propagate within their area, and don’t exit to other areas through the area border router (ABR)

For reference, the full summarisation command is (under OSPF process configuration):

area Z range X.X.X.X Y.Y.Y.Y not-advertise

Filter by Area

Filtering by summarisation is easy, but on occasion it can be too simple. Filtering by area allows a larger degree of customisation and control over route propagation.

OSPF area filtering is configured by using the command area X filter-list prefix Y with the last keyword being in for inbound filtering or out for outbound filtering. X represents the area number and Y represents the prefix name.

The prefix name is configured with the command ip prefix-list Y seq X following deny or permit then the prefix (For example 192.168.1.0/24)

Filter Locally

In some cases, routes need to be filtered locally on the router before they are installed into the routing information base. A distribution list can prevent routes being installed.

A distribution list is configured with the command distribute-list in the OSPF process configuration mode, followed by:

• An ACL number
• An ACL name
• A prefix with the keyword prefix then the prefix-list name
• A route-map with the keyword route-map then the route-map name

The command is finished with the final keyword of in for inbound or out for outbound