Unlike IGMPv1, in which the DR and the IGMP querier are typically the same device, in IGMPv2 the two
functions are decoupled. The DR and the IGMP querier are selected based on different criteria and may be
different devices on the same subnet. The DR is the device with the highest IP address on the subnet, whereas
the IGMP querier is the device with the lowest IP address.
Query messages are used to elect the IGMP querier as follows:
1
When IGMPv2 devices start, they each multicast a general query message to the all-systems group address
of 224.0.0.1 with their interface address in the source IP address field of the message.
2
When an IGMPv2 device receives a general query message, the device compares the source IP address in
the message with its own interface address. The device with the lowest IP address on the subnet is elected
the IGMP querier.
3
All devices (excluding the querier) start the query timer, which is reset whenever a general query message
is received from the IGMP querier. If the query timer expires, it is assumed that the IGMP querier has
gone down, and the election process is performed again to elect a new IGMP querier.
By default, the timer is two times the query interval.
Devices Running IGMPv3
IGMPv3 adds support for source filtering, which enables a multicast receiver host to signal to a device which
groups it wants to receive multicast traffic from, and from which sources this traffic is expected. This
membership information enables the software to forward traffic only from those sources from which receivers
requested the traffic.
IGMPv3 supports applications that explicitly signal sources from which they want to receive traffic. With
IGMPv3, receivers signal membership to a multicast group in the following two modes:
•
INCLUDE mode--In this mode, the receiver announces membership to a group and provides a list of
IP addresses (the INCLUDE list) from which it wants to receive traffic.
•
EXCLUDE mode--In this mode, the receiver announces membership to a group and provides a list of
IP addresses (the EXCLUDE list) from which it does not want to receive traffic. In other words, the host
wants to receive traffic only from sources whose IP addresses are not listed in the EXCLUDE list. To
receive traffic from all sources, like in the case of the Internet Standard Multicast (ISM) service model,
a host expresses EXCLUDE mode membership with an empty EXCLUDE list.
IGMPv3 is the industry-designated standard protocol for hosts to signal channel subscriptions in an SSM
network environment. For SSM to rely on IGMPv3, IGMPv3 must be available in the network stack portion
of the operating systems running on the last hop devices and hosts and be used by the applications running
on those hosts.
In IGMPv3, hosts send their membership reports to 224.0.0.22; all IGMPv3 devices, therefore, must listen to
this address. Hosts, however, do not listen or respond to 224.0.0.22; they only send their reports to that address.
In addition, in IGMPv3, there is no membership report suppression because IGMPv3 hosts do not listen to
the reports sent by other hosts. Therefore, when a general query is sent out, all hosts on the wire respond.
IGMPv3 Host Signalling
In IGMPv3, hosts signal membership to last hop routers of multicast groups. Hosts can signal group membership
with filtering capabilities with respect to sources. A host can either signal that it wants to receive traffic from
all sources sending to a group except for some specific sources (called exclude mode), or that it wants to
receive traffic only from some specific sources sending to the group (called include mode).
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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Configuring IGMP
IGMP Versions
