Lab 9.2.3 Identifying and Filtering Routes based on the NLRI

Objective:

Filter routing information using a distribute list in BGP.

Scenario:

Given the lab you just completed on Router Reflectors. You have decided that you want to prevent network 2.0.0.0 from being advertised to AS 200. You have decided to use a route filter on Router C.

Lab Tasks:

  1. Examine the routing table on Router D in AS 200. Do you see a BGP advertisement for network 2.0.0.0?
  2. If you do not see an advertisement for network 2.0.0.0, troubleshoot the network until you see the route in the BGP table.
  3. Log into router C. This is where we are going to filter the routing updates that go into AS 200. We are going to use a distribute list in order to accomplish this task.
  4. The first step in configuring a distribute list is to set up a standard access list. Since we are going to filter network 2.0.0.0 and allow all other networks, issue the following commands:
  • RouterC(config)#access-list 1 deny 2.0.0.0 0.255.255.255
  • RouterC(config)#access-list 1 permit 0.0.0.0 255.255.255.255

These commands are denying the 2.0.0.0 network while allowing any other network to pass freely.

  1. As with any access list statement, these commands do nothing until you enable them with the distribute list command. The command is listed below:
  • RouterC(config)#router bgp 100
  • RouterC(config-router)#neighbor 4.4.4.2 distribute-list 1 out
  1. Log onto Router D in AS 200 and issue the clear ip bgp * command.

Wait about 10 seconds and then issue the show ip bgp command. Did an entry for network 2.0.0.0 appear in the routing table?

Why do you think it is important to filter routing information?