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sonet, bgp, 2 isp's


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:


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.


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.




Re: sonet, bgp, 2 isp's

You could condition r2 and r3's default route on the existance of the connection to the internet being up from r1 and r4. The easiest way to do this is to receive the default from the isp, and redistribute it into the igp. When the connection dies, the default dies, and the igp will route the right way.

In fact, unless you are going to run ibgp on r2 and r3, then you won't gain much by getting full tables from the isp on both sides--you might as well just receive the default. If you want somewhat optimal routing, you can accept partial routes from the two providers, and run ibgp all the way through (full mesh or a reflector).


New Member

Re: sonet, bgp, 2 isp's


I started to think this through. If I static everything, then what is the point of receiving the full routes... good point.

Thanks for the help!


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