Swarm nodes can't see each other (IGMP snooping)

Question: My Swarm nodes can't see each other. What's wrong? Does anything have to be configured?

Answer: No configuration should be necessary as long as the nodes are on the same layer 2 broadcast domain, are configured in the same subnet, and are configured with the same multicast group address (224.0.10.100 by default). However, if your network switch has IGMP snooping enabled for the Swarm VLAN, it could be interfering with the multicast packets that are sent between the Swarm nodes.

The IGMP snooping feature on a network switch blocks the delivery of multicast packets to hosts that are not currently listening on the specific multicast address. When a host system, such as Swarm, begins listening on a multicast address, it sends an IGMP join notification to the router to let the router know that the host has begun listening. When a network switch implements IGMP snooping, it learns about hosts that are listening on a multicast address by snooping (eavesdropping) on the join notification and membership query responses. Some device in the subnet must implement the IGMP querier mechanism and that is either your gateway or your switch. The switch is appropriate in most cases.

If your switch is incorrectly blocking multicast from the Swarm nodes you can either:

Disable IGMP snooping on the Swarm VLAN, or
Enable an IGMP querier so that the switch will see the membership query responses.

Swarm's embedded operating system can use version 1, 2, or 3 for the IGMP packets. Swarm defaults to version 2 and the "igmpVersion" parameter is used to change it if necessary. So, for example, if you want Swarm to use IGMPv3 packets, you would set:

igmpVersion = 3

DataCore does not believe that IGMP snooping is a necessary feature to implement within most storage VLANs as the multicast traffic bandwidth is a small fraction of the overall traffic. Multicast signaling is the foundation of the Swarm clustering mechanism and it is crucial that all Swarm nodes receive the multicast transmissions from all other nodes.

If you think you are seeing this problem, you can confirm it by turning off IGMP snooping or plugging the nodes into a simple switch and see if the problem goes away.