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Leaving and Staying in Groups
The below illustration shows how multicast routers track and refreshes the state change in response to group-and-specific and
general queries.
● Host 1 sends a message indicating it is leaving group 224.1.1.1 and that the included filter for 10.11.1.1 and 10.11.1.2 are no
longer necessary.
● The querier, before making any state changes, sends a group-and-source query to see if any other host is interested in
these two sources; queries for state-changes are retransmitted multiple times. If any are interested, they respond with their
current state information and the querier refreshes the relevant state information.
● Separately in the below figure, the querier sends a general query to 224.0.0.1.
● Host 2 responds to the periodic general query so the querier refreshes the state information for that group.
Figure 50. IGMP Membership Queries: Leaving and Staying in Groups
IGMP Snooping
IGMP snooping is auto-configured on an Aggregator.
Multicast packets are addressed with multicast MAC addresses, which represents a group of devices rather than one unique
device. Switches forward multicast frames out of all ports in a VLAN by default, even if there are only a small number of
interested hosts, resulting in a waste of bandwidth. IGMP snooping enables switches to use information in IGMP packets to
generate a forwarding table that associate ports with multicast groups, so that the received multicast frames are forwarded only
to interested receivers.
How IGMP Snooping is Implemented on an Aggregator
● IGMP snooping is enabled by default on the switch.
● Dell Networking OS supports version 1, version 2 and version 3 hosts.
● Dell Networking OS — IGMP snooping is based on the IP multicast address (not on the Layer 2 multicast MAC address).
IGMP snooping entries are stored in the Layer 3 flow table instead of in the Layer 2 forwarding information base (FIB).
● Dell Networking OS — IGMP snooping is based on draft-ietf-magma-snoop-10.
● A maximum of 2k groups and 4k virtual local area networks (VLAN) are supported.
● IGMP snooping is not supported on the default VLAN interface.
● Flooding of unregistered multicast traffic is enabled by default.
● Queries are not accepted from the server side ports and are only accepted from the uplink LAG.
● Reports and Leaves are flooded by default to the uplink LAG irrespective of whether it is an mrouter port or not.
Disabling Multicast Flooding
If the switch receives a multicast packet that has an IP address of a group it has not learned (unregistered frame), the switch
floods that packet out of all ports on the VLAN. To disable multicast flooding on all VLAN ports, enter the no ip igmp snooping
flood command in global configuration mode.
When multicast flooding is disabled, unregistered multicast data traffic is forwarded to only multicast router ports on all VLANs.
If there is no multicast router port in a VLAN, unregistered multicast data traffic is dropped.
Internet Group Management Protocol (IGMP)
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