[c-nsp] BGP - Multihop across igp network

[Sascha E. Pollok]

Hello Paul,

> Connected to these 7600's we have a pair of 6500's not doing BGP (however
> participating in OSPF as all our boxes do)
>
> Connected off one of the 6500's we have a 3825 at a customer site
>
> The customer who is connected to a 3825 wants a full BGP feed from us.
>
> So, I created a pair of ebgp-multihop sessions between the 3825 and our two
> 7600's - both tables populated and everything looked good.  Once the
> customer brought up their session all the traffic seems to loop between our
> networks.. Almost like a blackhole effect.  In our 7600's I can see the
> customer routes advertising out and BGP itself looks correct when I do a
> "show ip bgp xxxxx" towards one of their netblocks.

Your other routers need to know about the customer's network.
You can either redistribute the customer's EBGP prefixes
into OSPF (selectively using e.g. route-maps on the 7600's)
or talk BGP to your other routes but learn only the customer's
routes. We are doing a similar thing. We are tagging customer-learned
routes with a BGP community and announce only prefixes carrying
this community via IBGP to the access routes.

Thus, the access routers carry only a few prefixes and can perfectly
route towards the customer.

Sascha