Users Guide

Layer 3 Routing Commands 1452
IGMPv3 is a major revision of the protocol and provides improved group
membership latency. When a host joins a new multicast group on an
interface, it immediately sends an unsolicited IGMP Report message for that
group.
IGMPv2 introduced a Leave Group message, which is sent by a host when it
leaves a multicast group for which it was the last host to send an IGMP
Report message. Receipt of this message causes the Querier possibly to
reduce the remaining lifetime of its state for the group, and to send a group-
specific IGMP Query message to the multicast group.The Leave Group
message is not used with IGMPv3, since the source address filtering
mechanism provides the same functionality.
IGMPv3 also allows hosts to specify the list of hosts from which they want to
receive traffic. Traffic from other hosts is blocked inside the network. It also
allows hosts to block packets for all sources sending unwanted traffic.
IGMPv3 adds the capability for a multicast router to learn which sources are
of interest to neighboring systems for packets sent to any particular multicast
address. This information gathered by IGMP is provided to the multicast
routing protocol (that is, DVMRP, PIM-DM, and PIM-SM) that is currently
active on the router in order to ensure multicast packets are delivered to all
networks where there are interested receivers.
IGMP mode is automatically enabled when PIM, DVMRP, or IGMP Proxy is
enabled.
ip igmp last-member-query-count
Use the ip igmp last-member-query-count command in Interface
Configuration mode to set the number of Group-Specific Queries sent before
the router assumes that there are no local members on the interface.
Syntax
ip igmp last-member-query-count Imqc
no ip igmp last-member-query-count
• Imqc — Query count. (Range: 1-20)
Default Configuration
The default last member query count is 2.