Using SET AS-PATH PREPEND

AS-PATH Prepending

The objective is to influence the best path selection of global BGP metrics,  in our topology  its AS 2 – AS 5.  In R5 reaching the AS 21 21.21.21.0 network is via R3 (AS2) which is only 64 Kbps. Manipulating the AS-PATH attribute in R2 will influence R5 to use the path going to R6.

Current best path without AS path attribute manipulation.

In R5 the best path to reach 21.21.21.0/24 network is via the next hop R3 (23.23.23.3) only 2 hops away.

Now let’s apply the route-map in R2 to influence R5 and other AS’s to use R6 as the next hop.

R2

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 network 21.21.21.0 mask 255.255.255.0

 neighbor 21.21.21.1 remote-as 1

 neighbor 22.22.22.3 remote-as 2

 neighbor 22.22.22.3 route-map prepend out

!

route-map prepend permit 10

 set as-path prepend 1 1

!

Verification:

Now the preferred path is changed to R6 (24.24.24.6) as the best path earlier is now showing an AS-PATH of 2 1 1 1 with the additional prepended path 1 1.