There are three
versions of IGMP, as defined by Request for Comments (RFC) documents of the
Internet Engineering Task Force (IETF). IGMPv2 improves over IGMPv1 by adding
the ability for a host to signal desire to leave a multicast group and IGMPv3
improves over IGMPv2 mainly by adding the ability to listen to multicast
originating from a set of source IP addresses only.
Table 1. IGMP Versions
basic query-response mechanism that allows the multicast device to determine
which multicast groups are active and other processes that enable hosts to join
and leave a multicast group. RFC 1112 defines the IGMPv1 host extensions for IP
allowing such capabilities as the IGMP leave process, group-specific queries,
and an explicit maximum response time field. IGMPv2 also adds the capability
for devices to elect the IGMP querier without dependence on the multicast
protocol to perform this task. RFC 2236 defines IGMPv2.
enabling a PIM on an interface enables IGMPv2 on that device. IGMPv2 was
designed to be as backward compatible with IGMPv1 as possible. To accomplish
this backward compatibility, RFC 2236 defined special interoperability rules.
If your network contains legacy IGMPv1 hosts, you should be familiar with these
operability rules. For more information about IGMPv1 and IGMPv2
interoperability, see RFC 2236, Internet Group Management Protocol, Version 2 .
Devices That Run
IGMPv1 devices send
IGMP queries to the “all-hosts” multicast address of 188.8.131.52 to solicit
multicast groups with active multicast receivers. The multicast receivers also
can send IGMP reports to the device to notify it that they are interested in
receiving a particular multicast stream. Hosts can send the report
asynchronously or in response to the IGMP queries sent by the device. If more
than one multicast receiver exists for the same multicast group, only one of
these hosts sends an IGMP report message; the other hosts suppress their report
In IGMPv1, there is
no election of an IGMP querier. If more than one device on the segment exists,
all the devices send periodic IGMP queries. IGMPv1 has no special mechanism by
which the hosts can leave the group. If the hosts are no longer interested in
receiving multicast packets for a particular group, they simply do not reply to
the IGMP query packets sent from the device. The device continues sending query
packets. If the device does not hear a response in three IGMP queries, the
group times out and the device stops sending multicast packets on the segment
for the group. If the host later wants to receive multicast packets after the
timeout period, the host simply sends a new IGMP join to the device, and the
device begins to forward the multicast packet again.
If there are
multiple devices on a LAN, a designated router (DR) must be elected to avoid
duplicating multicast traffic for connected hosts. PIM devices follow an
election process to select a DR. The PIM device with the highest IP address
becomes the DR.
The DR is
responsible for the following tasks:
register and PIM Join and Prune messages toward the rendezvous point (RP) to
inform it about host group membership.
host-query messages by default every 60 seconds in order to keep the IGMP
overhead on hosts and networks very low.
Devices That Run
IGMPv2 improves the
query messaging capabilities of IGMPv1.
The query and
membership report messages in IGMPv2 are identical to the IGMPv1 messages with
messages are broken into two categories: general queries (identical to IGMPv1
queries) and group-specific queries.
membership reports and IGMPv2 membership reports have different IGMP type
enhances IGMP by providing support for the following capabilities:
election process--Provides the capability for IGMPv2 devices to elect the IGMP
querier without having to rely on the multicast routing protocol to perform the
Response Time field--A new field in query messages permits the IGMP querier to
specify the maximum query-response time. This field permits the tuning of the
query-response process to control response burstiness and to fine-tune leave
Query messages--Permits the IGMP querier to perform the query operation on a
specific group instead of all groups.
messages--Provides hosts with a method of notifying devices on the network that
they wish to leave the group.
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:
devices start, they each multicast a general query message to the all-systems
group address of 184.108.40.206 with their interface address in the source IP
address field of the message.
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.
(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.