Re: Multicast in BGP/MPLS IP VPNs

[Yakov Rekhter <yakov at juniper.net>]

Yiqun,

> > Using multicast tree within the service provider(s) is one option
> > to carry VPN/VPLS customers' multicast traffic. But not the only
> > one feasible. Ingress replication is another feasible option.
> 
> Yakov,
> 
> (Please forgive me if I'm stating the obvious)
> 
> I think the point here is the balance between states and efficiency
> when delivering IP multicast packets to its intended receivers.
> 
> We all know that the best way to deliver IP multicast packets is to
> build multicast distribution trees.  

That would be true if minimizing bandwidth usage would be the only
objective.

In practice though, there are other objectives as well, such as
the amount of state and the overhead associated with maintaining
the state, operational complexity, etc... And if you take other
objectives into consideration, then building multicast trees may
not always be "the best way to deliver IP multicast packets".

>                                     PIM is the protocol that IETF has
> been standardizing to build such trees (inter- and intra- domain).
> One key advantage of PIM is it utilizes the network of routers to do
> the replication.  And different PIM variants (-SM, -SSM and bidir)
> offer different balance between state and efficient delivery.

... and all the current variants of PIM build shortest path trees, which
are known to be less efficient wrt bandwidth usage than minimum cost
trees. And, if efficient bandwidth use is an important issue, then
why should we be limited to shortest path trees ?

Yakov.