Administrator Guide

Internet Group Management Protocol (IGMP)

Internet group management protocol (IGMP) is a Layer 3 multicast protocol that hosts use to join or leave a multicast group.

Multicast is premised on identifying many hosts by a single destination IP address; hosts represented by the same IP address are 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.

IGMP Implementation Information

• Dell Networking Operating System (OS) supports IGMP versions 1, 2, and 3 based on RFCs 1112, 2236, and 3376, respectively.

• Dell Networking OS does not support IGMP version 3 and versions 1 or 2 on the same subnet.

• IGMP on Dell Networking OS supports an unlimited number of groups.

• Dell Networking systems cannot serve as an IGMP host or an IGMP version 1 IGMP Querier.

• Dell Networking OS automatically enables IGMP on interfaces on which you enable a multicast routing protocol.

Topics:

• IGMP Protocol Overview

• Congure IGMP

• Viewing IGMP Enabled Interfaces

• Selecting an IGMP Version

• Viewing IGMP Groups

• Adjusting Timers

• Preventing a Host from Joining a Group

• Enabling IGMP Immediate-Leave

• IGMP Snooping

• Fast Convergence after MSTP Topology Changes

• Egress Interface Selection (EIS) for HTTP and IGMP Applications

• Designating a Multicast Router Interface

IGMP Protocol 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 on version 1 by specifying IGMP Leave messages, which allows hosts to notify routers that they no longer care

about trac for a particular group.

Leave messages reduce the amount of time that the router takes to stop forwarding trac 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 trac.

To receive multicast trac from a particular source, a host must join the multicast group to which the source is sending trac. A host that

is a member of a group is called a receiver. A host may join many groups, and may join or leave any group at any time. A host joins and

366 Internet Group Management Protocol (IGMP)