EIGRP Route Filtering

• EIGRP is a distance vector routing protocol.
• The entire EIGRP topology is not known by each speaker like in Link State.
  • This means filtering and summarization can occur on any node in the network.

• Methods:
  • Distribute Lists, with Route-Maps
  • AD
  • Passive Interface
  • Distribute
  • Route Tags
  • Per Neighbor Prefix Limits

Distribute List Filter:
– Done with access-list and applying via distribute-list in topology base or router eigrp <as>

Same can be done with an IP prefix list as well. With the distribute-list command the name of the IP prefix list will be used instead of an access-list number. A prefix list will give more control over bit boundaries with the ge (greater than) or le (less than) option.

Administrative Distance:
– Setting the distance to 255 can stop routes from coming into routing table.
– In EIGRP this can be set under topology base.

First step is creating an access-list that matches the prefix that’s being filtered.

Then under topology base in EIGRP, setup the distance command.

255 is what the administrative distance value. 0.0.0.0 is the source, in this case we’re saying that route coming from anywhere. 255.255.255.255 is the wildcard for the source, and 2 is referencing the access list created.

Route Map with Tag:

Routes can be modified or labeled with a tag by using a route map set statement. This can then be used to filter or modify downstream via the tag value. Example below.

First step is to create a prefix list matching the prefix that’s going to be tagged.

Then a route map needs to be created that references the prefix list and uses the set tag option. In this example the tag is being set to 10.

The ‘route-map TAG permit 20’ is needed at the bottom to allow all other traffic to pass. There’s always an implicit deny at the end of each route map.

Next step is adding the route map to the EIGRP process.

TAG is the name of the route-map created, in is the direction we’re applying the route map, and Gig1 is the interface.

Now when doing a show ip route 3.3.3.3, there’s a tag value being applied.

The same tag value can be seen now on routers BranchA is advertising to downstream.

All of these features makes EIGRP very flexible. These types of route filtering and modification in other routing protocols often can only be completed during redistribution or when hitting an area border.