Internet Group Management Protocol (IGMP) | 211
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Internet Group Management Protocol (IGMP)
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 Snooping
• Fast Convergence after MSTP Topology Changes
• Designating a Multicast Router Interface
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.
To receive multicast traffic from a particular source, a host must join the multicast group to which the
source is sending traffic. 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 (Figure 12-1).
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