Administrator Guide

• 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 48. 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.
Internet Group Management Protocol (IGMP) 396