[c-nsp] Effect of IGMP version on IGMP Snooping

[John Neiberger]

We have a flat layer two network with IGMP snooping turned on. We are
currently running IGMP v2 everywhere, but we need to enable v3. We
tested this a few nights ago and found that switching to v3 caused a
disruption. Everything started working again after we switched back to
v2. This doesn't make much sense to me.

We have an interface acting as IGMP querier, so snooping seems to be
working. But why would changing IGMP versions cause any disruption?
And why would switching back to v2 suddenly make it work?

If it matters, this is a Cisco 4948 running 12.2(25).