3Com Switch 7750 Configuration Guide Guide

412 CHAPTER 43: IGMP CONFIGURATION
segment have members according to the returned IGMP report packets. The
multicast router also sends query packets periodically. When it receives the IGMP
join packets of a group member, it will refresh the membership information of the
network segment.
IGMP group-specific packets
The query router (querier for short) maintains the IGMP joins packets on the
interface on the shared network. After the related features are configured, the
IGMP querier will send IGMP group-specific query packets at the user-defined
interval for the user-defined times when it receives the IGMP leave packets from
the hosts.
Suppose a host in a multicast group decides to leave the multicast group. The
related procedure is as follows:
■ The host sends an IGMP leave packet.
■ When the IGMP querier receives the packet, it will send IGMP group-specific
query packets at the interval configured by the igmp
lastmember-queryinterval command (the interval is 1 second by default) for
the robust-value times (the robust-value argument is configured by the igmp
robust-count command and it is 2 by default).
■ If other hosts are interested in the group after receiving the IGMP
group-specific query packet from the querier, they will send IGMP join packets
in the maximum response time specified in the packet.
■ If the IGMP querier receives IGMP join packets from other hosts within the
robust-value x seconds time, it will maintain the membership of the group.
■ If the IGMP querier does not receive IGMP join packets from other hosts after
the robust-value x seconds time, it considers the group times out and will not
maintain the membership of the group.
The procedure is only fit for the occasion when IGMP queriers runs IGMP version
2.
If the host runs IGMP version 1, it does not send IGMP leave messages when
leaving a group, so the conditions will be the same as described in the procedure
above.
IGMP querier substitution rules
The lifetime of an IGMP querier is limited. If the former querier does not send
query messages in the specified time, another router will replace the IGMP querier.
The maximum query time of IGMP packets
When the host receives a query message, it will set a timer for each of its multicast
groups. The timer value is selected from 0 to the maximum response time at
random. When the value of a timer decreases to 0, the host will send the
membership information of the multicast group.
Through configuring the reasonable maximum response time, you can enable the
host to respond to the query information quickly and enable the Layer 3 switch to
understand the membership information of multicast groups quickly.