bgp ACL and LocPrf question

cisco steps · ‎07-08-2009

Can anyone please explain these commands to me? I am preparing for my BSCI exam and got stuck on this one learning BGP routing..

Here is the topology

R1-R2-R3-R4

(R2) to reach network 172.12.34.0 it needs to use next hope (R4) 10.1.1.4

(R2) to reach network 210.1.1.0 it needs to use next hope (R4) 10.1.1.4

Now we want (R2) to use (R3) as next hope to 172.12.34.0 and keep (R2) to reach network 210.1.1.0 trough (R4) 10.1.1.4

R1 (Config)# access list 18 permit 172.12.34 0.0.0.255

R1 (Config-router-map)#route-map PREFER _R3_FOR _172 permit 10

R1 (Config-router-map)#match ip address 18

R1 (Config-router-map)#set local-pref 200

R1 (Config-router-map)# route-map PREFER _R3_FOR _172 permit 20

R1 (Config-router-map)#set local-pref 100

I know we are trying to get the router to take a different route by creating ACL because that's what we preferred and changing local-pref to higer number will get the job done but something is not clear to me in the config and just needed some more explanation

Thanks

Istvan_Rabai · ‎07-08-2009

Hi Ocporburst,

A 0 is missing from your ACL. Corrected:

access list 18 permit 172.12.34.0 0.0.0.255

This config sets the local preference of the 172.12.34.0 prefix to 200 on R1.

Currently I don't see how it will make R2 to use R3 as the next-hop for this prefix.

Can you please clarify if the topology you have drawn represents also the BGP neighbor relationships between the routers.

Also, please indicate, which neighbor relationship is eBGP and which one is iBGP.

Thanks:

Istvan

Rui Carlos Antunes · ‎07-20-2009

Can you post the complete BGP configuration, so we can better understand the network and BGP relationships in question?