Enterasys 6000 User Guide

The following lists the features and functions supported when using IGMP:

Runs only when the switch is operating in the 802.1Q mode. 

Supports multiple multicast and non-multicast routers on the same VLAN.

Supports standalone multicast servers only if a router is present on the network.

Multicast forwarding rate is dependent on the number of ports to which the multicast stream is 
forwarded. More ports degrade the performance.

Supports IGMP version 1 and 2 only. Default is version 2.

Ports that are directly connected to routers are discovered dynamically through snooping for 
routing protocols.

IGMP is enabled or disabled per VLAN, and not per port.

IGMP will run with GMRP, however, the IGMP does have higher priority than GMRP.

The switch will “snoop” on all incoming multicast addresses to detect query responses, as well 
as queries. Query responses are sent to the multicast address detected in the stream from the host 
requesting to receive queries. The frame is an IP frame of protocol type 2. If the frame is a 
response frame, IGMP will take the multicast address and VLAN ID, and program a filter on the 
receive port. The response is then forwarded out upstream ports so that the router will receive it. 
Ports that receive queries are marked as upstream ports. It is assumed a router exists somewhere 
off this port, and responses are sent here. If the switch detects a router protocol on a port, that 
port is also marked as a router port. This keeps the switch from blocking traffic to other routers.

NOTE: Certain versions of firmware will not allow the switch to be a querier. Refer to 
the release notes shipped with your product and to standard (not shipped with product) 
RFC 2236, Section 8, for more information.