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between IP multicast routers / switches and IP multicast host groups to identify 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, and allows you to significantly reduce
the multicast traffic passing through your switch.
4.4.2.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.
Therefore, when DVMRP routing is enabled for a subnet on this switch, the switch will
automatically enable IGMP (chapter 2 “Configuring DVMRP” and chapter 3 “Configuring
DVMRP”).
4.4.3.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 similar join packet to all other ports on the switch, informing them that incoming
multicast traffic for the stated group is to be forwarded to the requesting port.