If your switch is MCAST aware, you should be able to keep mcast traffic on ports tagged for it to begin with. If your switch isn't mcast aware. you need to find a new switch.
MCAST aware means different things in different environments. Ideal is a switch that knows which multicast groups a particular port has joins on, rather than simply whether or not it is getting multicast traffic. In an ethernet fabric used as an exchange point, you have inter-AS multicast traffic, so sniffing IGMP doesn't do any good. Sniffing PIM sparse mode for joins would work. In some environments, you might be able to use RGMP to tell the switch which groups have been joined on a particular port (John Meylor mentioned over beer at the Cogent social that his group might consider writing up RGMP as an informational draft, so that number of enviornments may go up).
As for jumbo frames, will someone remind me what the benefit of using a larger MTU on the edges than you have in the core is? Is the edge device going to aggregate 6 1500-byte packets into a single 9000-byte jumbo frame for me?n
If it is not clear, I am talking about using jumbo frames on ethernet VLAN used in an exchange point; this would provide a migration path for service providers who have jumbo frames to the edge, because they could trade them over the exchange point frabric. They could, of course, do the same thing over a private interconnection. regards, Ted Hardie