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

[Christopher.Marget at usc-bt.com]

Peter said:
> 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.

It sounds like you might be describing a case where
- IGMP snooping is enabled on a switch
- There's no IGMP querier on the VLAN

If so, then we're in agreement.  That doesn't work (though it may appear to work at first).

There's a fourth option on some L2 switching platforms: enable an IGMP querier on the L2 device.  The queries don't have to come from anywhere sensible.*  As long as queries "just appear" on the LAN, clients will reply, and the IGMP snooping switch can eat the host reports that come in reply to his bogus queries.  No L3 interface required.

Definitely don't try to run IGMP snooping without a querier.

/chris

* There are rumors that some platforms won't respond to queries originated by 0.0.0.0.  No good reason for it.  I haven't run into it.  Maybe use a sensible address afterall.