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Designing BGP CE - Transit Network

Good afternoon everyone!

I've been looking for an explanation about this subject for a long time. When talking to other engineers, they don't really have a good solution for that.
So after a while, I decided to ask the specialists here! =)

First of all, let me start with my problem.

I'm a CE running BGP with some providers. Up to that point, everything is ok... The problem comes up when I have to decide what's the best approach for my transit network
(IP address used). By transit network I mean the address used between my routers running BGP and for example my Firewalls.

The topology would be like:

FW -      |
FW - L2 - iBGP
FW -      |
FW -      R - Providers

Let's suppose I received a block /20 from my provider and I decide to advertise addresses /23 or any other.
My question is: What's the best address to use in this L2 transit network? (Between my FWs and my Routers)

Let's suppose I use my /20 in this L2 network. To advertise more specific prefixes I'd have to create static routes pointing to the FWs for each network. However,
more than one FW are in the same /23, so I couldn't point my static routes to them. Another solution would be point my static routes to the interface, thus my FWs
would have to have Proxy ARP enabled. Another solution that I could use is use Secondary IP addresses on this interface connected to the L2 transit network.
All of it would be if I was using a /20 and trying to advertise more specific networks.

Is there any good other solution for this problem?
How about other designs? What would be the best one? Could anyone help me?


Thanks in advance.