Should I use default routes or full BGP table?

Question

I have a network with three links (ISPA, 3MB, ISPB, 1.5MB, ISPC, 50MB). I have three different speeds, and as such, ISPs A and B are primarily used for redundancy. I'm currently receiving routes from all three providers, but filtering a number of them on the backup links. As I would look to route traffic out the larger pipe near exclusively, I feel as though I do not need to retrieve a full routing table from all the ISPs, and could merely synchronize default routes.

Is this the case? Is there a way to limit BGP to only synchronizing next-hop or local routes? I'm worried about enabling full synchronization, as I'm running with older routers, and I'm working with limited amounts of RAM.

Answer 1

You could accept a full table from your main neighbour and ask the others to send you local routes + default route only. In that case, the full table routes would win over the default and your transit traffic would use the "full table" neighbour. traffic to the secondary neighbours should still use their uplinks as the ASPATH would be shorter. This only applies to outbound traffic though, if most of your traffic is inbound, you need to de-prioritise the routes advertised to the secondary neighbours (via AS-prepends, etc).

Answer 2

Unless you want to choose which ISP to use based on address, there is no reason to receive anything more than a default route. You can use the LOCAL-PREFERENCE attribute on the route received from ISP C to select it. Here's an example:

ip access-list standard DEFAULT-ONLY permit 0.0.0.0
!
route-map PREFERr-C permit 10
 match ip address DEFAULT-ONLY
 set local-preference 200
route-map PREFER-C permit 20
!
router bgp 9999
neighbor <isp c> route-map PREFER-C in
!

EDIT: When I read synchronizing routes, I realized your BGP routers were speaking to each other. In that case, you'll use local preference.