View Full Version : BGP - Advertising more specific routes

01-03-2010, 09:12 PM
Hello friends,

I have a problem at the moment, struggling to find a solution.

The situation is as follows:

20 remote sites all connecting into the same vpn mpls cloud running BDSL on the WAN.

All 20 sites have there own /24 LAN address.

What I'm trying to do is, advertise 2 x /25 via BGP into the cloud.

What is the best way to get BGP to install these routes, keeping in mind that the LAN is /24 so it will need an exact match to advertise the /25 into the BGP table.

Any help would be greatly appreciated.

01-03-2010, 09:43 PM
If you point two static routes at the PE routers toward the real /24 subnet, and redistribute those and not the /24 I think that should do the trick.

You want to be careful not to blackhole (half) your traffic, but if you influence administrative distances that shouldn't be a problem.

What you want to do doesn't really make any sense, though, unless you are prepping for a future re-ip'ing of the subnets. Just a thought, keeping in mind that I don't know your network or what you're trying to accomplish.

01-05-2010, 05:07 AM
Hi Friend,

There are 2 ways to do this:

First you can use BGP path injection (on the router that receiving the BGP updates, it will split the /24 into /25's then you can do AS-Prepend or anything else).

Second way is to use policy maps to advertise /25 from the advertising router, but you will loose 2 IP addresses for this setup to work.

for example, you have and you want to advertise /25 subnet, such as, you will configure your router to have and secondary address of then use a prefix list like the following:
ip prefix-list permit 10 MY_PREFIX
ip prefix-list permit 20 MY_PREFIX

Hope that helps.

01-12-2010, 04:14 AM

The routes are now installed into the route table. I realized that I need to learn more on BGP.

Thanks, Chris, David.