Brian,
If you have full meshed IBGP sessions then r1 will automatically get the
route to 12.X.X.X in case link to UUNET failed. Or you have to use r1 and r2
as the route reflectors.
Hope this helps,
J.
  brian kastor  wrote: All,

I have 2 sites. We currently get full routing tables from uunet and AT&T
into a single router. We want to put a second router at another site and
run IBGP between just those two. We will have two routers between the two
running BGP. I guess it would look something like this:

site1-r1(bgp)---r2---r3---r4(bgp)-site2

R1 will get UUnet, r4 will get at&t. R1(UUNet) is being used as primary. 
Behind R1 is our HQ. Behind r4 is a large branch.
My concern would be if the link to UUnet goes down, how does traffic from
site 1 get out r4.

Scenario:
User pings 12.x.x.x This is unknown on site 1, so it goes to r1. UUnet is
down, so the route is pointing to r4 (ibgp). The packet still has a
destination address of 12.x.x.x, but how does r2 know what to do with this
packet? It doesn't know where 12.x.x.x is?

Someone here at work brought this up to me and I keep saying that the ibgp
session gets established, so when you look at the table on r1 for 12.x.x.x,
the next hop is the interface of r4. He keeps saying, 'yeah, but when that
packet gets to r2, the destination is 12.x.x.x, which it doesn't know.'

All of this can be solved if someone tells me that we just need to run ibgp
on all 4 routers and i shouldn't see a performance hit. I am worried
because these routers are going to be routing user traffic between the sites.

Thanks,

bk
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