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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 100 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.
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