Igmp Basics - Cisco Nexus 7000 Series Configuration Manual

IGMP Basics

• Hosts no longer perform report suppression, which means that hosts always send IGMP membership
For detailed information about IGMPv2, see
For detailed information about IGMPv3, see
IGMP Basics
The basic IGMP process of a router that discovers multicast hosts is shown in the figure below. Hosts 1, 2,
and 3 send unsolicited IGMP membership report messages to initiate receiving multicast data for a group or
channel.
Figure 9: IGMPv1 and IGMPv2 Query-Response Process
In the figure below, router A, which is the IGMP designated querier on the subnet, sends query messages to
the all-hosts multicast group at 224.0.0.1 periodically to discover whether any hosts want to receive multicast
data. You can configure the group membership timeout value that the router uses to determine that no members
of a group or source exist on the subnet. For more information about configuring the IGMP parameters, see
Configuring IGMP Interface Parameters.
The software elects a router as the IGMP querier on a subnet if it has the lowest IP address. As long as a router
continues to receive query messages from a router with a lower IP address, it resets a timer that is based on
its querier timeout value. If the querier timer of a router expires, it becomes the designated querier. If that
router later receives a host query message from a router with a lower IP address, it drops its role as the
designated querier and sets its querier timer again.
In this figure, host 1's membership report is suppressed and host 2 sends its membership report for group
224.1.1.1 first. Host 1 receives the report from host 2. Because only one membership report per group needs
to be sent to the router, other hosts suppress their reports to reduce network traffic. Each host waits for a
random time interval to avoid sending reports at the same time. You can configure the query maximum
response time parameter to control the interval in which hosts randomize their responses.
Cisco Nexus 7000 Series NX-OS Multicast Routing Configuration Guide
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• The multicast state that is maintained for groups and sources, not just for groups as in IGMPv2.
reports when an IGMP query message is received.
RFC 2236.
RFC 3376.
Configuring IGMP