3Com 3C17203 - SuperStack 3 Switch 4400, Implementation Manual

Page: 41 / 153

Multicast Filtering 41
Figure 8 The effect of multicast filtering
Multicast Filtering
and Your Switch
Your Switch provides automatic multicast filtering support using IGMP
(Internet Group Management Protocol) Snooping. It also supports IGMP
query mode.
Snooping Mode
Snooping Mode allows your Switch to forward multicast packets only to
the appropriate ports. The Switch “snoops” on exchanges between
endstations and an IGMP device, typically a router, to find out the ports
that wish to join a multicast group and then sets its filters accordingly
Query Mode
Query mode allows the Switch to function as the Querier if it has the
lowest IP address in the subnetwork to which it belongs.
IGMP querying is disabled by default on the Switch 4400. This helps
prevent interoperability issues with core products that may not follow the
lowest IP address election method.
You can enable or disable IGMP query mode for all Switch units in the
stack using the
queryMode
command on the Command Line Interface
IGMP menu.
You would enable query mode if you wish to run multicast sessions in a
network that does not contain any IGMP routers (or queriers). This