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Prefix List Exercises

February 25, 2013

Prefix lists are a powerful tool for route filtering but they also may be a little confusing when learning them. Here are some prefix list examples to reinforce knowledge. Answers should be syntactically sound commands and using the most specific matching possible.

  1. Filter the following routes:

* 10.1.5.1/24
* 10.2.2.1/24
* 10.5.7.1/24

  1. Filter the following routes:

* 10.1.1.1/24
* 10.2.1.1/25
* 10.2.1.128/25
* 10.100.1.1/30

  1. Filter all but the last route and allow no others

* 172.16.2.1/24
* 172.16.3.1/25
* 172.16.3.128/25
* 172.16.4.1/26
* 172.16.4.64/26
* 172.16.10.1/30

  1. Filter all routes using one command

* 10.1.1.1/16
* 10.2.1.1/17
* 10.2.128.1/17
* 10.10.10.5/30

  1. Filter all /30 routes

* 192.168.1.0/24
* 192.168.2.1/25
* 192.168.2.128/30
* 192.168.3.1/30

Answers are below so don’t scroll down too far!

Answers

1.

ip prefix-list 1 deny 10.0.0.0/13 ge 24 le 24
ip prefix-list 1 permit 0.0.0.0/0 le 32

2.

ip prefix-list 2 deny 10.0.0.0/8 ge 24 le 30
ip prefix-list 2 permit 0.0.0.0/0 le 32

3.

ip prefix-list 3 deny 172.16.0.0/20 ge 24 le 26
ip prefix-list 3 permit 172.16.10.0/30

4.

ip prefix-list 4 deny 10.0.0.0/12 ge 16 le 30
ip prefix-list 4 permit 0.0.0.0/0 le 32

5.

ip prefix-list 5 deny 192.168.2.0/23 ge 30 le 30
ip prefix-list 5 permit 0.0.0.0/0 le 32

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From → Exercises, Lab

3 Comments
  1. Matteo permalink

    Hi, i am trying to understand IP Prefix-list and i am finding your website very useful. However i don’t understand the second example, why the first statement is ip prefix-list 2 deny 10.0.0.0/9 ge 24 le 30 rather than 10.0.0.0/8 ge 24 le 30? Thank You so much for your help

    • Kevin Breit permalink

      Sorry about the long delay. You are correct and I fixed the answer.

  2. Raul permalink

    The answer to the 3rd question seems incorrect.
    Since the last prefix should be allowed, I believe we should not take it into consideration when calculating the prefix for the networks to be blocked. Therefore a block of 8 in the 3rd octet (/21) will include the necessary ones, so believe the correct answer is :

    ip prefix-list 3 deny 172.16.0.0/21 ge 24 le 26
    ^
    ip prefix-list 3 permit 172.16.10.0/30

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