Enterasys Networks 1H582-xx User Manual

The Internet Group Management Protocol (IGMP) runs between hosts and their immediately 
neighboring multicast switch device. The protocol’s mechanisms allow a host to inform its local 
switch device that it wants to receive transmissions addressed to a specific multicast group.

A multicast-enabled switch device can periodically ask its hosts if they want to receive multicast 
traffic. If there is more than one switch device 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 switch device can determine 
which (if any) multicast traffic needs to be forwarded to each of its ports. At Layer-3, multicast 
switch devices use this information, along with a multicast routing protocol, to support IP 
multicasting across the Internet.

IGMP provides the final step in an IP multicast packet delivery service since it is only concerned 
with forwarding multicast traffic from the local switch device to group members on a directly 
attached subnetwork or LAN segment.

This switch device supports multicast group management by

passively snooping on the IGMP query and IGMP report packets transferred between IP 
multicast switches and IP multicast host groups to learn IP multicast group members, and

actively sending IGMP query messages to solicit IP multicast group members.

The purpose of multicast group management is to optimize a switched network’s performance so 
multicast packets will only be forwarded to those ports containing multicast group hosts or 
multicast switch devices instead of flooding to all ports in the subnet (VLAN).

In addition to passively monitoring IGMP query and report messages, the Matrix E1 can also 
actively send IGMP query messages to learn locations of multicast switches and member hosts in 
multicast groups within each VLAN.

However, note that IGMP neither alters nor routes any IP multicast packets. Since IGMP is not 
concerned with the delivery of IP multicast packets across subnetworks, an external IP multicast 
switch device is needed if IP multicast packets have to be routed across different subnetworks.

IGMP VLAN Registration (IVR) is designed for applications using wide-scale deployment of 
multicast traffic. For example, the broadcast of multiple television channels over a campus network 
or multi-tenant environment. IVR allows a user on a port to subscribe and unsubscribe to a multicast 
stream on the network-wide multicast VLAN, using IGMP open mode. It allows the single