Enterasys Networks 1G58x-09 - Manual (Page 469)

About IGMP
IGMP VLAN Registration
Matrix E1 Series (1G58x-09 and 1H582-xx) Configuration Guide 10-13
10.3 ABOUT IGMP
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.
10.3.1 IGMP VLAN Registration
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