Igmp and flooding on switches

[carl_townshend]

Hi all

i know that switches have igmp  enabled by default, what happens to multicast traffic when this is the case? Is it flooded to all ports or not?

what happens in the case of no pim router on querier? Do they drop or forward still ?

cheers

[Jon Marshall]

The multicast range 224.0.0.x is link local and flooded anyway. 

For the rest, if there is no querier then multicast is treated as broadcast or at least it is on most switches (I seem to remember a case on here where without a querier it is just dropped). 

Jon

[Joseph W. Doherty]

"i know that switches have igmp enabled by default, . . ."

You mean IGMP snooping, correct?

There's no standard for IGMP snooping, as it's proprietary, although RFC4541 make recommendations, so implementations might vary between switches that support it. (NB: Interestingly, the RFC authors queried different vendors, see section #4.)

Cisco's default (I believe [however, Jon's memory might be better than mine]), an IGMP snooper would "block" multicast traffic, that it normally would, unless it "knows", via IGMP queries, the port wants it.

If this is a problem, such as just wanting to do L2 only multicast, same L2 domain, w/o an IGMP querier, this might be dealt with by disabling IGMP snooping (which of course would flood multicast to all ports) or, if supported, enabling a switch to be an IGMP querier. (For the latter, you only need one switch, if you have more than one in the same L2 domain, for this function.)