[c-nsp] inter-as multicast

[Shanawaz]

MSDP + MBGP has worked well for us.

MBGP does the routing + tcp connection + rpf checks. It uses the multicast
address family extension of BGP and configs are similar to your unicast BGP
configs.

MSDP connects a chosen RP from your domain to another RP from your peer
domain to exchange multicast sources. Ofcourse you can apply filters to
limit what groups you want to exchange information for. We use the GLOP
range (233.x.x.x) and filter everything else

HTH

Shanawaz