[c-nsp] BGP multihop between two sites

[Cord MacLeod]

On Sep 2, 2009, at 1:51 PM, Randy McAnally wrote:

> We have two sites advertising unique subnets via the same AS.  Since  
> the
> subnets originate from the same AS they apparently get dropped from  
> the tables
> at each site.
>
> Each site has at least 3 upstreams taking full tables from each with  
> a 6509.
>
> Site a has a single border router with dual supervisors.
>
> Site b actually has dual border routers.  Each router handles  
> different
> upstreams and trades routes via iBGP.
>
> So I think I need to set up a multihop session between the sites.
>
> Right now, traffic between sites is taking the floating default  
> route and
> causes issues between sites when one of our BGP peers is down for  
> whatever reason.
>
> What are the things to look out for when setting up this multihop?   
> Are there
> gotcha's to deal with given one site has dual active routers?

Multihop BGP is not a good idea in this example.  You'll want to use  
something like neighbor x.x.x.x allowas-in on your eBGP sessions.   
However, you'll also want to setup a BGP filter to deny your own  
sourced prefixes to prevent loops/flapping.  Meaning, when you allow  
prefixes from site A into site B, setup a BGP filter on site B denying  
B's prefixes and vice versa.