Confused about mVPN and MDT

I don't know why, but this seems to be a subject that just doesn't make
sense to me. I'm having trouble visualizing the process and I don't yet
understand all of the steps necessary to configure mVPN.

One thing I'm not entirely sure of is why we would even configure this in
the first place. My only thought is that it is a service to provide to
customers and it saves them from having to configure GRE tunnels between
their sites since regular L3VPN wouldn't support multicast. I don't see why
they couldn't just configure GRE tunnels between their CE routers and run
PIM on them. I guess it would work, but the idea of being a service
provider is to offload some of that headache, right?

Okay, to the technical stuff. I'll walk through the process as I understand
it and you'll see where I'm getting lost. Let's assume that the basic L3VPN
config is done and working.

1. PIM SM is enabled on P/PE routers and an RP is selected.
2. PIM SM is configured in the customer network and will show as a PIM
neighbor inside their VRF on our PE router.
3. The MDT address is configured in the VRF.
4. The ipv4 mdt address family is configured in BGP with the same peers as
in the VPNv4 AF.

Here's where I get confused. At some point in this process, the PE routers
join the MDT group. They also form GRE tunnels between their loopbacks.
This seems to be an automatic process. I don't understand the need for the
MDT group. If the PE routers already have GRE tunnels between them, PIM
could run across them with no problem as-is. What function does the MDT
perform and what benefit does it provide?

Another question is this: what actually triggers the building of those GRE
tunnels? Is that a function of enabling the ipv4 mdt address family?

Please un-confuse me. If anyone has any helpful pointers or even old blog
posts, please point the way.

Thanks!
John

Blogs and organic groups at http://www.ccie.net
Received on Tue Jan 08 2013 - 20:40:54 ART