SMC Networks SMC6724L3 manual Igmp Protocol, 135

MULTICAST FILTERING

This switch not only supports IP multicast filtering by passively monitoring IGMP query and report messages and multicast routing probe messages to register end-stations as multicast group members, but also supports the DVMRP and PIM-DM multicast routing protocols required to forward multicast traffic to other subnets (page 3-222and 3-231).

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 or PIM, to support IP multicasting across the Internet.

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