Lab 122 - BGP AS-Path Access-List

Prerequisites: CCNP level skills.

Note!
Topology from Lab 118.

Topology

Pic. 1 - Topology Diagram.

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

Pic. 2 - Regular Expressions Quick Review.

Task 1
R4 seems to be advertising prefixes that it does not originate making AS 40 a transit autonomous system. Configure R4 so that it only advertises its own prefixes.

Task 2
On R5 filter out all prefixes originated by AS 40.

Solution

Task 1

R4 seems to be advertising prefixes that it does not originate making AS 40 a transit autonomous system. Configure R4 so that it only advertises its own prefixes.

Pic. 3 - Prefixes Advertised by R4 to R1.

Pic. 4 - Prefixes Advertised by R4 to R2.

Notice!
R4 is advertising prefixes originated by other autonomous systems.

R4 Configuration:

!

ip as-path access-list 1 permit ^$
!

router bgp 40
 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
 redistribute connected route-map CONN_TO_BGP
 neighbor 10.1.14.1 remote-as 10
 neighbor 10.1.14.1 filter-list 1 out
 neighbor 10.1.24.2 remote-as 20
 neighbor 10.1.24.2 filter-list 1 out
 no auto-summary

!

Verification:

Pic. 5 - Prefixes Advertised by R4 to R1.

Notice!
R4 is advertising only its own prefixes now.

Task 2

On R5 filter out all prefixes originated by AS 40.

Pic. 6 - R5 Receives Prefixes Originated by AS 40.

R5 Configuration:

!

ip as-path access-list 1 deny _40$
ip as-path access-list 1 permit .*
!
router bgp 50
 no synchronization
 bgp router-id 172.16.105.5
 bgp log-neighbor-changes
 network 172.16.105.0 mask 255.255.255.0
 neighbor 10.1.35.3 remote-as 30
 neighbor 10.1.35.3 filter-list 1 in
 no auto-summary

!

Clear ip bgp * in

Verification:

Pic. 7 - R5's BGP Table.