As-Prepend config

[getaway51]

Hi,

May i try to understand wht these prepend command means in layman term?

Why adding ip prefix-list Prepend-list-1 and ip prefix-list Prepend-list-2 below? 

R1274-R1

no ip prefix-list Prepend-list-1 seq 1 permit 10.17.5.0/24

ip prefix-list Prepend-list-1 seq 5 permit 172.16.1.0/27

ip prefix-list Prepend-list-1 seq 10 permit 172.16.1.128/27

R1274-R2

no ip prefix-list Prepend-list-2 seq 1 permit 172.16.2.0/24

no ip prefix-list Prepend-list-2 seq 2 permit 172.16.1.0/24

ip prefix-list Prepend-list-2 seq 5 permit 172.16.1.32/27

[sunil.krnetwork1]

AS-Path is one of the BGP attribute to influence the path selection for outbound traffic .

Lets say you want  the outbound traffic to flow from primary Link instead of secondary Link , then you will do a path prepend on the secondary link  (  Increasing multiple same  AS number to the path ). the traffic flow be from the primary link .

[getaway51]

Hi,

Does it means tht for config above in R1274-R2, "ip prefix-list Prepend-list-2 seq 5 permit 172.16.1.32/27" means traffic destined to  172.16.1.32/27 will goes through R1, Since the prepend command is in R2?

Theoretically wht happen?

[Georg Pauwen]

Hello,

without seeing what is actually prepended (can you post that as well) it remains a bit of guesswork, but the most likely result of the prefix lists is this:

172.16.1.32/27 will go through R1, and 172.16.1.0/27 and 172.16.1.128/27 will go through R2.

[getaway51]

Hi,

What is the command needed to see the result mentioned?

Thanks!

[Georg Pauwen]

Hello,

post the output of:

show route-map

[Richard Burts]

I believe that the original poster is confused about what he has. He says this " Since the prepend command is in R2?". But I do not see any prepend command in either of them. What is in the post is the configuration of a prefix list which has the name Prepend-list-1 and Prepend-list-2. It is likely that the prefix lists are used in route maps which are part of neighbor configuration in BGP. The prefix list will identify what prefixes will get prepended. But the prefix list does not itself do any prepending. If we are going to continue this discussion we will need to see all of the BGP configuration of these routers.