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traffic, and allows 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 adjacent 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 if 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 role of querying
the LAN for group members. It then propagates the service
requests on to any adjacent multicast switch/router to ensure that
it will continue to receive the multicast service.
Based on the group membership information learned from IGMP, a
router/switch can determine which (if any) multicast traffic needs
to be forwarded to each of its ports. At Layer 3, multicast routers
use this information, along with a multicast routing protocol such
as DVMRP, to support IP multicasting across the Internet.
Note that IGMP neither alters nor routes IP multicast packets. A
multicast routing protocol must be used to deliver IP multicast
packets across different subnetworks. GMRP Protocol
GARP Multicast Registration Protocol (GMRP) allows network
devices to register endstations with multicast groups. GMRP
requires that any participating network devices or endstations
comply with the IEEE 802.1p standard. Compliant endstations can
request to receive traffic from a multicast group simply by issuing
a join packet that includes a known multicast address. When the
join packet reaches a port on the switch, it configures this port to
receive multicast traffic for the requested group, and then issues a
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