Advertise second default route via BGP, and have one site prefer it over the primary

[HOLLYW00D]

Howdy,

Here's what I'm trying to achieve:

We have an MPLS cloud (five sites).  Routes being advertised via BGP.

Site 1 is advertising "network 0.0.0.0"

Site 1 gets its default route from an ASA via EIGRP

All five sites currently route to Site 1 for 0.0.0.0/0

I'd like to advertise a second(ary) default route from Site 2, and I want Site 3 to prefer this default route over the one advertised by Site 1.  However, I want Site 4 and Site 5 to continue preferring the original default route from Site 1.

To configure Site 2, I will be using a static route for 0.0.0.0/0 instead of EIGRP, but I don't have enough experience to pull off the rest.  Hoping someone can help guide me in the right direction.  Thank you in advance!

[Richard Burts]

You have not told us much about your BGP environment. Do all 5 routers peer with each of the other routers (full mesh)? Or do the other routers just peer with site 1 (hub and spoke)? If it is a full mesh you might use communities to achieve your goal. Site 1 would assign a community on its default route and site 2 would assign a different community on its advertisement of the default route. Site 3 would have a route map that looks at the community and prefers the one from site 2 while site 4 and 5 would have route maps that looks at the community and prefers the one from site 1.

HTH

Rick

[HOLLYW00D]

Thank you, Rick!  Yes, it's a full mesh BGP environment.

Your solution sounds logical, albeit daunting since all sites will need to be reconfigured.

The purpose of this is to slowly migrate all sites to Site 2 for the default route.  I wanted to test/create a proof of concept with one site using the new route, without affecting the other sites.

Would it be possible for Site 2 to advertise the second default route with a long AS-path, and then somehow configure Site 3 to prefer that route over the shorter AS-path route?

Thanks again!

[Richard Burts · Accepted solution]

Yes it would be possible to configure site 2 to prepend its advertisement of the default route (and would require a route map to accomplish this). You would need a route map on site 3 to prefer the advertisement with the longer AS path. And if you are going to gradually move sites 4 and 5 to use site 2 advertisement then you will need route maps on those sites also. So either way you will need route maps on most of the sites.

Based on the additional information you have provided it occurs to me that there is another alternative to consider. Since this is a full mesh BGP you should be able to use the weight attribute to influence which advertised default route to use. Site 1 and site 2 would simply advertise their default route. Site 3 would use weight to prefer the advertisement from site 2 while site 4 and 5 would use weight to prefer the advertisement from site 1. And as you want to gradually move sites to use site 2 you simply change their weight configuration. This would be more simple than my first suggestion and should work just fine.

HTH

Rick

[Richard Burts]

I am glad that my suggestions were helpful. Thank you for marking this question as solved. This will help other readers in the forum to identify discussions that have helpful information.

HTH

Rick