Dell M6220 User Configuration Manual page 706

in scenarios, such as FCoE/FIP snooping, where it is desirable to flood L2
control plane multicast messages that do not utilize well-known multicast
addresses.
By default, multicast routers are aged out every five minutes. The user can
control whether or not multicast routers age out. If all multicast routers age
out, the switch floods the VLAN with the multicast group.
Multicast routers send an IGMP query every 60 seconds. This query is
intercepted by the switch and forwarded to all ports in the VLAN. All hosts
that are members of the group answer that query. The switch intercepts the
replies and forwards only one report per group from all of the received
responses.
In summary:
• IGMP snooping controls the flooding/forwarding behavior for multicast
groups. Multicast data is flooded in the VLAN until a multicast router port
is identified.
• IGMP snooping is enabled by default
• Multicast filtering is enabled by default
• IGMP snooping forwards multicast sources to multicast routers by default
• Reserved multicast IP addresses (224.0.0.x) are always flooded to all ports
in the VLAN
• Unregistered multicast traffic may be flooded in the VLAN by a user
configuration option.
NOTE:
It is strongly recommended that users enable MLD snooping if IGMP
snooping is enabled and vice-versa. This is because both IGMP snooping and
MLD snooping utilize the same forwarding table and not enabling both may
cause unwanted pruning of protocol packets utilized by other protocols, e.g.
OSPFv3.
IGMP Snooping Querier
When PIM and IGMP are enabled in a network with IP multicast routing, the
IP multicast router acts as the IGMP querier. However, if the IP-multicast
traffic in a VLAN needs to be Layer 2 switched only, an IP-multicast router is
not required. The IGMP snooping querier can perform the role of generating
IGMP queries that would normally be performed by the multicast router.
706
Configuring L2 Multicast Features