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 a
rea 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
permit then the prefix (For example
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
prefixthen the prefix-list name
- A route-map with the keyword
route-mapthen the route-map name
The command is finished with the final keyword of
in for inbound or
out for outbound