[c-nsp] Load Sharing multihomed BGP with two providers

[andrew2 at one.net]

Trying to figure the best way to accomplish this:

I have two routers each connected to a different provider, each taking
full routes from their respective providers.  Currently all outbound
traffic is directed to Router A, so long as it is available and
announcing itself via IGP.  Router A has a default route that points to
Router B.  Since the routes learned from Provider A are more specific
than the default route, currently 99.99% of outbound traffic goes out
Provider A unless there is a problem with Router A or Provider A.  Which
is how I wanted it.

Now, however, I'd like to share some the outbound load with Provider B
on Router B.  I'd like to get the full routes from both providers on
both routers, turn on bgp multipath and see how things go.  I'm just
unsure on what the best way to get those routes into the routers is.
One idea I've been kicking around is to check with both providers and
see if they'll allow a second BGP session via ebgp multihop sessions.
In this scenario I'd be sure to inject the next_hop from each provider
into IGP and limit the max hops so that if the circuit to a provider
goes down the BGP session doesn't try to establish over the other
provider's circuit.  Any major gotchas in this scenario?  Is this
something the big "Tier 1's" typically permit?

Any better way of doing this?  Route Reflector?  iBGP?  Redistributing
the routes into an IGP seems evil, but technically possible...

Thanks for any pointers,

Andrew