Route Filtering with Distribute Lists

Routers running routing protocols send updates to indicate network reachability. At some point, you will have to filter those updates. Why? First,to hide certain networks from the rest of the organization. Second, to control network overhead on the wire. Cisco IOS provides many tools to control/filter routing updates and one of them is the Distribute List.

Distribute Lists

Distribute Lists allow you to filter routing updates through an access list. You can filter routing updates coming in or going out of the router. Be reminded though that distribute lists work differently on link-state routing protocols (OSPF and IS-IS). 

For example, RB will send out all updates from RA to RC except for the 192.168.1.0/24 network.

Without Distribute List:

With Distribute List

Step 1. Create an access list

We need to create an access list that will deny the 192.168.1.0/24 network and still allow the other networks to pass through.

Step 2. Go to the routing process and configure the distribute list

We then go to the EIGRP routing process and configure the distribute list. The distribute list will match the addresses in access-list 99 and filter them as they go out of the serial 1/1 interface.

To check inbound and outbound filtering of routing updates use the show ip protocols command.

 Now let's check the routing table of the three routers.

 

RB has filtered the 192.168.1.0/24 network and it doesn't appear in RC's routing table.