Lab 9.2.5 Prefix List

Objectives:

  • Use a prefix list to filter routes.

Scenario:

Router B is receiving advertisements about network 172.50.0.0 and 172.60.0.0 from Router C in AS 100. Router B should only be able to advertise 172.0.0.0/8 to AS 300

Tasks:

  1. Cable and configure the lab as shown in the diagram. Use BGP as your routing protocol. On Router-C, advertise both networks 172.50.0.0 and 172.60.0.0 with the network command. You will also need to advertise the aggregate-address 172.0.0.0 255.0.0.0 on Router-C. Don't forget to advertise network 192.168.10.0 on Router A also. Before moving on to the next step, be sure that Router-A and Router-C are seeing each others routes with the show ip route and show ip bgp commands.

  2. Look at the routing table of Router A, what routes do you see?
  3. To permit only 172.0.0.0/8 updates, we are going to configure a prefix list called supernetonly. To do this, issue the following command:

    Router-B(config)# ip prefix-list supernetonly permit 172.0.0.0/8

  4. Just like an access-list, this has no effect until we assign it to a neighbor. To assign the prefix list to Router-A, issue the following command:

    Router-B(config-router)#neighbor 10.10.2.2 prefix-list supernetonly out

  5. Clear your BGP routing table on Router-A with the clear ip bgp * command and examine the routing table of Router-A. What routes do you see now?