Multicast routers use IGMP snooping to learn which groups have members on their attached physical networks. A multicast router keeps a list of multicast group memberships for each attached network and a timer for each membership. In a multicast group membership, at least one member of a multicast group on an attached network is available.
In response to the request, the host creates an entry in its Layer 2 forwarding table for that VLAN. When other hosts send join requests for the same multicast, the device adds them to the existing table entry. Only one entry is created per VLAN in the Layer 2 forwarding table for each multicast group.
VLANs can be configured as snooping only or routing with snooping. When Layer 3 multicast routing is enabled on a VE, snooping for the underlying VLAN is enabled implicitly. Both explicit and implicit snooping can be enabled on a VLAN. Implicit snooping is the default. When explicit snooping is disabled on a VE that has routing enabled, snooping reverts back to implicit snooping. This does not change the functionality, but only removes the configuration.
When routing is disabled on a VE where explicit snooping is configured, the routing side of the programming stops and the snooping side of the programming takes over. When routing is enabled, the Layer 3 IGMP querier takes precedence on that VLAN. When routing is disabled and the snooping querier is configured, then the snooping querier takes effect.