Alcatel Carrier Internetworking Solutions 8008 Multicast Filtering, Igmp Snooping, Igmp Protocol

Multicast Filtering

Multicast Filtering

Multicasting sends data to a group of nodes instead of a single destination. The simplest way to implement multicasting is to broadcast data to all nodes on the network. However, such an approach wastes a lot of bandwidth if the target group is small compared to overall the broadcast domain.

Since applications such as video conferencing and data sharing are more widely used today, efficient multicasting has become vital. A common approach is to use a group registration protocol that lets 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 OmniStack® 8008 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 its 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 responsibility of querying the LAN for group members.

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 any IP multicast packets. A multicast router/ switch must be used to deliver IP multicast packets across different subnetworks.

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