Flood Unknown Multicast Traffic Switching

[mattmar]

Looking for a more detailed description of what Meraki MS switch defines as unknown multicast traffic. Is it as simple as any traffic that did not create state in the snooping table with IGMP join/report messages? In other words devices just sending to groups but not participating in IGMP process would fall in the unknown traffic.. Would device sending L2 mcast mac 0100:5e00.xxxx.xxxx fall in unknown. etc..

[m_Andrew]

Hello,

You are correct. A switch treats a received multicast packet as unknown when the destination group is one for which the switch has not observed an explicit group join (signaled via an IGMP report packet).

If flood-unknown is enabled, such packets will get forwarded out all ports (except the ingress port). If flood-unknown is disabled, then the packet would only get forwarded out the ports where an IGMP querier or MRD advertiser is known. If none are known, the packet will be discarded by the switch.

[TEAM-ind]

Older post, I know, but I have a related question:  Do default multicast settings apply to all switches and stacks in the network?  Or do you need to add each stack and define the settings?

Circled red is the default setting, and circled blue is a stack within the network.  For another stack in the network to have these settings, do I need to add that stack and apply the same settings?

[jnekl]

The default multicast settings will apply to every switch and stack, unless the switch/stack in question already has its own settings applied (non-default). 

So, in your example, the blue circled stack will use whatever is configured for the settings there, ignoring the default. Every other switch/stack will use the default unless they as well get their own settings like the blue circle. Hope that clears it up.

[TEAM-ind]

That is what I thought.  However, I do have a load balancer that uses multicast sitting on the other stack in this network, and it does not seem to be communicating.  While the stack circled blue needed to be added in order to allow multicast from an application called Singlewire, even though the default settings were the same.  If I add the other stack that is in this network with the same settings, do I need to do that in a maintenance window?  I am wondering if making these changes may cause interruptions to any other traffic on the network that is not related to multicast.

[jnekl]

I wouldn't expect that change to cause any interruption with non-mulitcast traffic, but I of course recommend making any change in a change window just to be safe.