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ip igmp snooping static

The command ip igmp snooping static allows me to statically join a layer-2 interface to a multicast group. I suppose I do this if my host needs to see the multicast stream, but cannot generate IGMP for some reason. But does it also generate IGMP for the group towards the multicast router port? If not, I cannot see how useful it could be.

I suppose if the switch is also L3, then you could do an ip igmp join-group on the VLAN interface, but on a L2-only switch, that is not an option.

Same thing applies, I guess, if IGMP snooping is switched off and replaced with static CAM table entries.

I also notice that IGMP snooping only affects traffic with ethertype 0x0800, i.e. IP. Any frames with other ethertypes, e.g. 0x886F from a WLBS server, get flooded, even if they have a 01:00:5e prefix.

Anyone got any thoughts on these observations?

Kevin Dorrell

Luxembourg

2 REPLIES

Re: ip igmp snooping static

Anyone?

Cisco Employee

Re: ip igmp snooping static

Hello Kevin,

The ip igmp snooping static command does not generate an igmp join to the multicast router port. What it does do is create a static cam entry in the snooping table. In reality that is how snooping works, it programs cam entries (for the mcast address) into the cam table.

The igmp snooping process puts the cam entry in there by looking at the destination address of the join (vs in unicast looking at the source address to create a cam entry)

As you said, removing snooping, and enabling a static cam primarily does the same thing, however the snooping process no longer looks at this packet.

Unfortunately we cannot constrain the non ip packets with ethertype of 0x886F.

Hope this helps

Thank you

Matt

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