IGMP querying without multicast routing

IGMP snooping may not be available on the 4948. Snooping by design is passive, it only listens for IGMP messages to determine what multicasts need to be enabled on a port. IGMP snooping is also a layer 2 service, and having an IP address on a VLAN is not a requirement to make it work.

Hmmm.

A routed interface on the same VLAN (anywhere on the VLAN) is in fact required for igmp. The exception being that "ip igmp snooping querier" allows a L2 box to substitute for the normal L3 interface.

I would be astonished if the 4948 did not do igmp snooping.

If you don't have a sperate router maybe:- you could configure L3 on the 4948 but turn off "IP routing". Then just maybe you would get the required response from the L3 interfaces.

no ip routing int vl x ip address x.x.x.x ip pim sparse ! maybe you need or don't need this.

Thanks.

IGMP snooping is certainly available on this box. In fact, it's enabled by default (as I said).

The problem isn't that I can't get snooping to work -- it works fine.

I can't get *querying* to work. Having an IP address on the VLAN (probably) is required to send queries. They're IP packets afterall. They have to have a source address.

/chris

That's getting very close to what I'm looking for: The minimal config to enable querying, but without forwarding any packets from one VLAN to another.

And I think I may have stumbled onto the answer, but am unable to test anymore right now: ip multicast-routing ...but without pim (or any other routing protocol) enabled on any of the interfaces.

I think that's enough to get the supporting infrastructure (including querying) up and running, but with no routing protocols, the packets can't go anywhere.

Thanks very much!

/chris

I have no idea what you are trying to do here, but I will take a stab to help you. If the source and destination multicast devices are on the same VLAN, all you need it IGMP snooping enabled. (You need to read up on what IGMP is and why IGMP snooping is required in a switching environment) Multicast routing and IP routing are needed if the source and destination devices are on different VLANs. If everything is on the same switch/router you won't need a routing protocol or PIM, but routing will be turned on and traffic routed between VLANs because all the VLANs are directly connected.

Scott

I'm trying to make a 4948 send IGMP queries.

If I want IGMP snooping to *work*, your recipe is one ingredient short. It also needs an IGMP querier. Without queries, there will be no IGMP traffic beyond the initial host report, so IGMP snooping will have nothing to eavesdrop upon.

Oh my.

Sending IGMP queries is a *router* function. It's part of the IGMP2 specification, and is definitely not an the job of an L2 switch. Right now my switch (even with an interface on the VLAN) is just an L2 switch.

It appears that I need to enable some subset of multicast-routing functionality in order to get the switch to send queries. I just want to enable only the minimum functionality so that multicast traffic doesn't get routed between VLANs.

This question really isn't a protocol question. It's a Cisco implementation question.