Lab 106 - BGP Filtering using Prefix-List

Prerequisites: CCNP level skills.

Topology

Pic. 1 - Topology Diagram.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
On R4 check what prefixes are originated by AS 50.

Task 2
On R4 filter out the following:

• Class A prefixes shorter than /24
• Class C prefixes longer than /27

Use prefix-list to accomplish this.

Solution

Task 1

On R4 check what prefixes are originated by AS 50.

The command to use: show ip bgp regexp _50$

Pic. 2 - BGP Table on R4

(prefixes originated by AS 50)

Task 2

On R4 filter out the following:

• Class A prefixes shorter than /24
• Class C prefixes longer than /27

Use prefix-list to accomplish this.

The following should be filtered out:
Class A prefixes shorter than /24 = 5.5.0.0/16
Class C prefixes longer than /27 = 200.55.55.0/29

R4 Configuration:

!

ip prefix-list MATCH_TASK_2 seq 5 deny 0.0.0.0/1 le 23
ip prefix-list MATCH_TASK_2 seq 10 deny 192.0.0.0/3 ge 28
ip prefix-list MATCH_TASK_2 seq 15 permit 0.0.0.0/0 le 32
!

router bgp 124
 no synchronization
 bgp router-id 172.16.104.4
 bgp log-neighbor-changes
 network 172.16.104.0 mask 255.255.255.0
 network 172.16.144.0 mask 255.255.255.0
 neighbor 10.1.14.1 remote-as 124
 neighbor 10.1.14.1 route-reflector-client
 neighbor 10.1.14.1 prefix-list MATCH_TASK_2 in
 neighbor 10.1.24.2 remote-as 124
 neighbor 10.1.24.2 route-reflector-client
 neighbor 10.1.24.2 prefix-list MATCH_TASK_2 in
 no auto-summary

!

Verification:

Pic. 3 - BGP Table on R4

(prefixes originated by AS 50)

The following prefixes have been filtered out:

Class A prefixes shorter than /24 = 5.5.0.0/16
Class C prefixes longer than /27 = 200.55.55.0/29