Users 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 14. 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.
Internet Group Management Protocol (IGMP)
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