6.4 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 the overall broadcast domain.

Since applications such as video conferencing and data sharing are 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 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).

The switch routing switch supports IP multicast filtering not only by passively monitoring IGMP Query and Report messages and DVMRP Probe messages to register end-stations as multicast group members (Layer 2), but also by actively sending GMRP Query messages to learn the location of multicast routers/switches and member hosts in multicast groups within each VLAN (Layer 3). This switch also supports the DVMRP multicast routing protocol required to forward multicast traffic to other subnets.

6.4.1 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.

6.4.2 IGMP Protocol

The Internet Group Management Protocol (IGMP) runs between hosts and their immediately neighboring multicast router/switch. IGMP is as 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 is 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. It then propagates the service requests on to any neighboring 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 any 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.

WGS3 Layer 3 Switch User’s Manual

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