iPod

Dec 30 2008

Prefix Lists

IP Prefix lists can be used with BGP to permit or deny specific prefixes from being advertised or learnt to or from a neighbor.

Consider the topology below.

We will carry out three exercises.

1. configure a prefix list to match 192.168.1.0/24
2. configure a prefix list to match 192.168.1.0/24, 192.168.1.0/25, 192.168.1.0/26.
3. configure a prefix list to match 192.168.1.0/25 and 192.168.1.0/26

Exercise 1

We configure the following prefix list and attach it to the bgp neighbor 10.0.0.2 using the commands below.

ip prefix-list slash-24-only seq 5 permit 192.168.1.0/24

router bgp 1
!
neighbor 10.0.0.2 prefix-list slash-24-only in

Exercise 2

We configure the following prefix list and attach it to the bgp neighbor 10.0.0.2 using the commands below.

ip prefix-list UP-TO-SLASH-26 seq 5 permit 192.168.1.0/24 le 26

router bgp 1
!
neighbor 10.0.0.2 prefix-list UP-TO-SLASH-26 in

Exercise 3

We configure the following prefix list and attach it to the bgp neighbor 10.0.0.2 using the commands below.

ip prefix-list GE-LE seq 5 permit 192.168.1.0/24 ge 25 le 26

router bgp 1
!
neighbor 10.0.0.2 prefix-list GE-LE in

Now let me try and explain what these 3 prefix lists are actually doing.

Prefix list 1 – slash-24-only

ip prefix-list slash-24-only seq 5 permit 192.168.1.0/24

This is pretty straight forward.  This prefix list will match on the exact prefix as configured in the prefix list ie 192.168.1.0/24.

Prefix list 2 – UP-TO-SLASH-26

ip prefix-list UP-TO-SLASH-26 seq 5 permit 192.168.1.0/24 le 26

For a prefix to be permitted by this prefix-list the first 24 bits must match the first 24 bits of 192.168.1.0.

The le 26 then adds a subnet clause which states that the subnet mask being advertised must be less than or equal to 26 bits in length.

Lets consider a bunch of prefixes and see if they would be permited by the above prefix list.

1. 192.168.1.0/24
2. 192.168.1.4/30
3. 192.168.1.128/25
4. 192.168.1.0/23
5. 192.168.1.0/27

Prefix 1 matches both criteria ie the first 24 bits in prefix 1 match the prefix in the prefix list and the subnet mask is less than 26 bits.

Prefix 2 matches the first criteria ie the first 24 bits in prefix 1 match the prefix in the prefix list, however the subnet mask is greater than 26 bits.

Prefix 3 also matches both criteria ie the first 24 bits in prefix 1 match the prefix in the prefix list and the subnet mask is less than 26 bits.

Prefix 4 is an invalid prefix, I’ll let you work out why.

Prefix 5 matches the first criteria but fails on the subnet mask length criteria and as such the prefix is denied.

Prefix list 3 – GE-Le

ip prefix-list GE-LE seq 5 permit 192.168.1.0/24 ge 25 le 26

Now this is an interesting beast.  This prefix-list had 2 match clauses.

1. The prefix must match the first 24 bits on the prefix in the prefix list
2. The subnet mask must be between 25 and 26 bits in length

Lets consider a bunch of prefixes and see if they would be permited by the above prefix list.

1. 192.168.1.0/24
2. 192.168.1.4/30
3. 192.168.1.128/25

Prefix 1 matches the first criteria ie the first 24 bits match, however the subnet mask is the wrong length.

Prefix 2 matches the first criteria ie the first 24 bits match, however the subnet mask is the wrong length.

Prefix 3 matches both criteria ie the first 24 bits in prefix 1 match the prefix in the prefix list and the subnet mask is greather than 25 bits buts still less than 26 bits.

I hope that makes sense.  Anyhow, here is a video showing how to put it all together or alternativley click here to download and watch it on your iPod.

By zismail • BGP, CCIE, CCIE SP, CCIP, CCNP, iPod, Security • 0 • Tags: CCIE, CCIE SP, Security