Table 5: IGMP Versions
DescriptionIGMP Version
Provides the 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 multicasting.
IGMPv1
Extends IGMP, 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.
IGMPv2
By default, 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 .
Note
Devices That Run IGMPv1
IGMPv1 devices send IGMP queries to the “all-hosts” multicast address of 224.0.0.1 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
messages.
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:
•
Sending PIM register and PIM Join and Prune messages toward the rendezvous point (RP) to inform it
about host group membership.
•
Sending IGMP host-query messages.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
36 OL-32598-01
Configuring IGMP
IGMP Versions Differences
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