igmp snooping

[umetsu]

If the switch suppots the "igmp" function,the multicast traffic sends to the port that multicast receiver is connected. But,if the switch does NOT support "igmp" function,multicast traffic sends to the all ports.I think that it looks same as broadcast traffic. Is it right ? What's the difference on this situation ? Please let me know asap.

[Richard Burts]

From the perspective of the switch, if it is not doing igmp snooping or cgmp then every multicast will be forwarded to every port belonging to the VLAN. This is the same behavoir as it does for broadcast. So to the switch they are treated alike.

From the perspective of the end station connected to the switch port they are quite different. A broadcast packet is always something the NIC is listening for and the NIC will receive the broadcast frame and generate an interrupt to process the frame. If it is a multicast and the NIC is not listening for that multicast MAC then the NIC does not generate an interrupt and there is no impact to the end station of the multicast while there is always an impact to the end station from broadcast.

HTH

Rick

[Kevin Dorrell]

What Rick says is all correct, but I would like to add a bit more about IGMP snooping, which is enabled by default on most switches.

When an end-station wants to listen to a multicast, it generates an IGMP report (which is, itself, a multicast) every 60 seconds to reassure the source that there is somebody listening. If the source does not see any IGMP reports, it is allowed to stop sending.

IGMP snooping exploits this conversation to control which switchports get the multicast stream. Very simply, with IGMP snooping, multicasts are initially all blocked, except the IGMP reports themselves. When the switch sees an IGMP report coming from an end station, it opens the multicast stream to that port so that the end station can hear the whole stream.

This IGMP processing, of course, is done by the processor. But the IGMP is a multicast. So, when the switch receives a multicast, how does it know whether to wake up the processor? That's where "hardware support for IGMP snooping" comes in. With such hardware support, only IGMPs wake up the processor, and they only happen once a minute. The hardware support recognises an IGMP in hardware, and wakes up the processor. The multicast data itself is forwarded by the normal cam-table hardware.

The implication here is that IGMP snooping is only advisable if your switch supports it in hardware. If it doesn't, then every multicast packet has to wake up the processor in case it is an IGMP, and that makes the CPU take a dive. Luckily, even the low-end 2950 supports IGMP snooping in hardware, so the processor only gets woken up once a minute.

There is only one thing to add to this: multicast routers. Your multicast router has got to receive all the multicast stream data regardless of IGMP snooping. So the switch also listens for PIM hellos (by default); if it hears them, it open the port to all multicast. (There are other ways to recognise the multicast router: static configuration, CGMP, but PIM hellos is the default.)

That's it in a nutshell. Hope it was interesting.

Kevin Dorrell

Luxembourg