[c-nsp] Need help with setting up ip multicastrouting

[Christopher.Marget at usc-bt.com]

Gert Doering said:
> On Tue, Oct 12, 2010 at 07:45:10PM +0200, Peter Rathlev wrote:
> > I'm just guessing here,
> > but I don't see why routed multicast traffic would need to have the L2
> > group bit set.
> 
> Well, if you only ever have two routers in a subnet, it doesn't need to...
> 
> But what if you have 20 routers in a subnet and want the multicast traffic to
> be heard by only those 5 that are interested in it?

If those routers' interfaces are multicast enabled, an IGMP snooping switch will flood multicast traffic to them, regardless of interest in a particular group.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800b0871.shtml

The switch listens to the following messages in order to detect router ports with IGMP snooping:
· IGMP Membership query send to 01−00−5e−00−00−01
· PIMv1 hello send to 01−00−5e−00−00−02
· PIMv2 hello send to 01−00−5e−00−00−0d
· DVMRP probes send to 01−00−5e−00−04
· MOSPF message send to 01−00−5e−00−05 or 06
By enabling IGMP snooping on a switch, all the above MAC entries are added to the show cam system
command output of the snooping switch. Once a router port is detected, it is added to the port list of all GDAs
in that VLAN.

/chris