Deployment Guide
Internet Group Management Protocol (IGMP)
On an Aggregator, IGMP snooping is auto-congured. 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
• Joining and Filtering Groups and Sources
• Leaving and Staying in Groups
• 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 trac for a particular group. Leave messages reduce the amount of time that the router takes to stop forwarding trac 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 trac.
To receive multicast trac from a particular source, a host must join the multicast group to which the source is sending trac. 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
leaves a multicast group by sending an IGMP message to its IGMP querier. The querier is the router that surveys a subnet for multicast
receivers and processes survey responses to populate the multicast routing table.
IGMP messages are encapsulated in IP packets which is as illustrated below:
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