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Free CCNP GNS3 Lab IP Prefix-List





Cisco IOS prefix lists work similar access lists for route advertisements (prefixes). While extended, and to a limited extent, standard access lists can be configured to match prefix announcements, prefix lists are far more powerful. Like an access lists; a prefix list contains one or more ordered entries which are processed sequentially. As with access lists, the evaluation of a prefix against a prefix list ends as soon as a match is found.

Additional keywords like “GE” and “LE” can be added to the prefix-list, the “len” value changes its meaning. When using GE and LE, the len value specifies how many bits of the prefix you are checking, starting with the most significant bit.

Most CCNP students find prefix-lists challenging so this lab is an effort to help you understand how prefix-lists work by using them as route filters. We will build a simple network consisting of four Cisco routers and a switch, after which we will configure an IP prefix-last using the keywords “GE” and “LE” to filter IP routing.

Learning Objectives:

  • Review basic router configuration.
  • Review basic BGP configuration.
  • Understand and configure IP prefix-list.
  • Understand and configure keywords “GE” and “LE”.
  • Apply IP prefix-list to filter routing.

Basic Configuration:

  1. Configure IP addresses and subnet mask on Loopback interfaces of all routers as shown in the network diagram.
  2. Configure IP addresses and subnet mask on Fastethernet interfaces of all routers as shown in the network diagram.
  3. Configure IP addresses and subnet mask on Serial interfaces of all routers as shown in the network diagram.
  4. Insure that all Fastethernet and serial interfaces are not administrative down.

BGP Configuration:

  1. Enable BGP routing protocol on R1 as autonomous system 65101.
  2. Configure R2 and R3 as BGP neighbors.
  3. Enable BGP routing protocol on R2 as autonomous system 65102.
  4. Configure R1 as BGP neighbors.
  5. Advertise the 172.16.10.0/24 network.
  6. Advertise the 172.16.11.0/24 network.
  7. Enable BGP routing protocol on R3 as autonomous system 65103.
  8. Configure R1 as BGP neighbors.
  9. Display routing table on R3

R3#sh ip route bgp

172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks

B 172.16.10.0/24 [20/0] via 10.1.1.1, 00:03:56

B 172.16.11.0/24 [20/0] via 10.1.1.1, 00:03:56

B 172.16.0.0/16 [20/0] via 10.1.1.1, 00:03:56

Prefix-List Configuration:

  1. On R2 create an IP prefix-list to filter the 172.16.0.0/16 network to include only the 172.16.10.0/24 and 172.16.11.0/24 networks.
  2. Apply the IP prefix-list to the BGP routing on R2.
  3. Display the routing table on R3. 

R3#sh ip route bgp

172.16.0.0/24 is subnetted, 2 subnets

B 172.16.10.0 [20/0] via 10.1.1.1, 00:03:19

B 172.16.11.0 [20/0] via 10.1.1.1, 00:03:19

Note: the 172.16.0.0/16 route no longer exist in the routing table.

This conclude this Free CCNP GNS3 Lab.

Download this lab now to see more:

  BGP prefix-list (531.1 KiB, 1,235 hits)


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One Response to “Free CCNP GNS3 Lab IP Prefix-List”

  1. Change your prefix-list so that only the default-route is allowed, as a result only the default route should be in Vanilla s routing table.

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