Enterasys Networks 9034313-07 manual Igmp Configuration, Igmp Overview

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IGMP Configuration

This chapter describes the IGMP Configuration set of commands and how to use them.

For information about...	Refer to page...
IGMP Overview	13-1
Configuring IGMP at Layer 2	13-2
Configuring IGMP on Routing Interfaces	13-10

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).

In addition to passively monitoring IGMP query and report messages, the SecureStack C3 can also actively send L3 IGMP query messages to learn locations of multicast devices 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, multicast routing is needed if IP multicast packets have to be routed across different subnetworks.

SecureStack C3 Configuration Guide 13-1