Microsoft ES4649, ES4625 manual Multicast Filtering, Igmp Protocol

Multicast Filtering 3

Multicast Filtering

Multicasting is used to support real-time applications such as videoconferencing or streaming audio. A multicast server does not have to establish a separate connection with each client. It merely broadcasts its service to the network, and any hosts that want to receive the multicast register with their local multicast switch/ router. Although this approach reduces the network overhead required by a multicast server, the broadcast traffic must be carefully pruned at every multicast switch/router it passes through to ensure that traffic is only passed on to the hosts which subscribed to this service.

Unicast Flow

Multicast Flow

This switch uses IGMP (Internet Group Management Protocol) to query for any attached

hosts that want to receive a specific multicast service. It identifies the ports containing hosts

requesting to join the service and sends data out

to those ports only. It then propagates the service request up to any neighboring multicast switch/router to ensure that it will continue to receive the multicast service. This procedure is called multicast filtering.

The purpose of IP 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 traffic to all ports in the subnet (VLAN).

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 3-264and 3-271).

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.

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