Administrator Guide

Internet Group Management

Protocol (IGMP)

On an Aggregator, IGMP snooping is auto-configured. You can display information on IGMP by using show

ip igmp

command.

Multicast is based on identifying many hosts by a single destination IP address. Hosts represented by the same

IP address are a multicast group. The internet group management protocol (IGMP) is a Layer 3 multicast

protocol that hosts use to join or leave a multicast group. Multicast routing protocols (such as protocol-

independent multicast [PIM]) use the information in IGMP messages to discover which groups are active and

to populate the multicast routing table.

This chapter contains the following sections:

• IGMP Overview

• IGMP Snooping

Topics:

• IGMP Overview

• IGMP Version 2

• IGMP Version 3

• IGMP Snooping

• How IGMP Snooping is Implemented on an Aggregator

• Disabling Multicast Flooding

• Displaying IGMP Information

IGMP Overview

IGMP has three versions. Version 3 obsoletes and is backwards-compatible with version 2; version 2 obsoletes

version 1.

IGMP Version 2

IGMP version 2 improves upon version 1 by specifying IGMP Leave messages, which allows hosts to notify

routers that they no longer care about traffic for a particular group. Leave messages reduce the amount of

time that the router takes to stop forwarding traffic for a group to a subnet (leave latency) after the last host

leaves the group. In version 1 hosts quietly leave groups, and the router waits for a query response timer

several times the value of the query interval to expire before it stops forwarding traffic.

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