Lab 125 - BGP Local-AS No-prepend Replace-AS

Prerequisites: CCNP level skills.

Note!
Configuration from Lab 123 must be used in this lab.

Topology

Pic. 1 - Topology Diagram.

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

Task 1
After merging the three ASes (Lab 123/124), the administrator of AS 50 noticed that prefixes arriving on R5 carry the AS 123 in the AS_PATH attribute. Before AS 50 configure their R5 router to peer with AS 123, the administrator asked you to remove AS 123 from the AS_PATH attribute.

Solution

Task 1

After merging the three ASes (Lab 123/124), the administrator of AS 50 noticed that prefixes arriving on R5 carry the AS 123 in the AS_PATH attribute. Before AS 50 configure their R5 router to peer with AS 123, the administrator asked you to remove AS 123 from the AS_PATH attribute.

Pic. 2 - Current BGP Table on R5.

R3 Configuration:

!

router bgp 123
 no synchronization
 bgp router-id 172.16.103.3
 bgp log-neighbor-changes
 network 172.16.103.0 mask 255.255.255.0
 neighbor 10.1.13.1 remote-as 123
 neighbor 10.1.13.1 route-reflector-client
 neighbor 10.1.13.1 next-hop-self
 neighbor 10.1.23.2 remote-as 123
 neighbor 10.1.23.2 route-reflector-client
 neighbor 10.1.23.2 next-hop-self
 neighbor 10.1.35.5 remote-as 50
 neighbor 10.1.35.5 local-as 30 no-prepend replace-as
 no auto-summary

!

Pic. 3 - R5's BGP Table.

Notice!
The option 'no-prepend' removes the local-as numbers (look at lab 124). The option 'replace-as' strips off the original AS number when updates are sent to the BGP neighbor configured to use the local-as number.