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

[Peter Rathlev]

On Wed, 2010-10-13 at 06:08 -0700, David Barak wrote:
> There is a document on cisco's site regarding how IGMP snooping
> breaks multicast in typical LAN environments.  I don't have the
> link handy, but it should be googleable.  Effectively, the issue is
> that the switches do not have a way to properly identify the mrouter
> port, and end up cutting off legitimate flows.

I don't think this is directly relevant for routing multicast. The
problem is that a switch with IGMP snooping enabled will only forward
multicast frames to other hosts on the same switch _unless_ it has an
mrouter port for that VLAN.

The solution to this is either:

 1) Disable IGMP snooping for the VLAN, thereby forfeiting the
    advantages of less flooding.

 2) Enable IGMP Snooping Querier on a L3 interface on some device
    (doesn't matter which) on this VLAN.

 3) Enable PIM on a L3 interface on some device on this VLAN.

If you're implementing multicast routing option 3 is the natural choice.
If you only want to enable cross-switch multicast switching option 2
means you can avoid configuring PIM.

(This was something I learned the hard way recently. :-])

-- 
Peter