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

[John Neiberger]

On Tue, Oct 12, 2010 at 11:45 AM, Peter Rathlev <peter at rathlev.dk> wrote:
> On Tue, 2010-10-12 at 17:08 +0200, Gert Doering wrote:
>> On Tue, Oct 12, 2010 at 10:50:09AM -0400, Steven Pfister wrote:
>> > That makes sense... igmp snooping seems to be enabled, but it still
>> > isn't working. Is there something else that needs to be done to the
>> > 2960?
>>
>> Since there is no IGMP between routers, IGMP snooping can not be
>> helpful here.  Chance is that it's actually *eating* the multicast
>> packets and waiting for IGMPs to show up to tell it where to send
>> which packets...
>
> I don't know much about multicast, but aren't routed multicast packets
> sent as L2 unicast between multicast routers? I'm just guessing here,
> but I don't see why routed multicast traffic would need to have the L2
> group bit set.
>
> I that is the case then a switch in between two PIM interfaces shouldn't
> need to know anything about multicast.
>
> --
> Peter

That's correct. IGMP is only necessary on the switches connected to
the end devices. If there are any L2 switches in the PIM path, they
will be remain blissfully unaware of the details.

John