Gigabit Ethernet Switch Management Guide

A DVANCED T OPICS
4-7
nodes join or leave multicast groups. A switch or router can then
easily determine which ports contain group members and send data out
to those ports only. This procedure is called multicast filtering.
The purpose of multicast filtering is to optimize a switched network’s
performance, so multicast packets will only be forwarded to those
ports containing multicast group hosts or multicast routers/switches
instead of flooding to all ports in the subnet (VLAN). The
TigerSwitch 1000 supports multicast filtering by passively monitoring
IGMP Query and Report messages.
IGMP Snooping
A Layer 2 switch can passively snoop on IGMP Query and Report
packets transferred between IP Multicast Routers/Switches and IP
Multicast host groups to learn the IP Multicast group members. It
simply monitors the IGMP packets passing through it, picks out the
group registration information, and configures multicast filters
accordingly. IGMP Snooping generates no additional network traffic,
allowing you to significantly reduce the multicast traffic passing
through your switch.
IGMP Protocol
The Internet Group Management Protocol (IGMP) runs between
hosts and their immediately neighboring multicast router/switch.
IGMP is a multicast host registration protocol that allows any host to
inform its local router that it wants to receive transmissions addressed
to a specific multicast group.
A router, or multicast-enabled switch, can periodically ask their hosts
is they want to receive multicast traffic. If there is more than one
router/switch on the LAN performing IP multicasting, one of these
devices is elected “querier” and assumes the responsibility of querying
the LAN for group members.