Enterasys Networks D-Series - Manual (Page 363)

Enterasys D-Series CLI Reference 13-1
13
IGMP Configuration
This chapter describes the IGMP Configuration set of commands and how to use them.
IGMP Overview
About IP Multicast Group Management
The Internet Group Management Protocol (IGMP) runs between hosts and their immediately
neighboring multicast device. The protocol’s mechanisms allow a host to inform its local device
that it wants to receive transmissions addressed to a specific multicast group.
A multicast‐enabled device can periodically ask its hosts if they want to receive multicast traffic. If
there is more than one 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 device can determine which (if
any) multicast traffic needs to be forwarded to each of its ports. At Layer‐3, multicast devices use
this information, along with a multicast routing protocol, to support IP multicasting across an IP
network.
IGMP provides the final step in an IP multicast packet delivery service, since it is only concerned
with forwarding multicast traffic from the local device to group members on a directly attached
subnetwork or LAN segment.
This device supports IP multicast group management by passively snooping on the IGMP query
and IGMP report packets transferred between IP multicast devices and IP multicast host groups to
learn IP multicast group members.
The purpose of IP 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 devices instead of flooding to all ports in the subnet (VLAN).
About Multicasting
Multicasting is used to support real‐time applications such as video conferences 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
For information about... Refer to page...
IGMP Overview 13-1
Configuring IGMP at Layer 2 13-2