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.
•
Sending 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
IGMPv2 improves the query messaging capabilities of IGMPv1.
The query and membership report messages in IGMPv2 are identical to the IGMPv1 messages with two
exceptions:
•
IGMPv2 query messages are broken into two categories: general queries (identical to IGMPv1 queries)
and group-specific queries.
•
IGMPv1 membership reports and IGMPv2 membership reports have different IGMP type codes.
IGMPv2 also enhances IGMP by providing support for the following capabilities:
•
Querier 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 process.
•
Maximum 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 latencies.
•
Group-Specific Query messages--Permits the IGMP querier to perform the query operation on a specific
group instead of all groups.
•
Leave-Group messages--Provides hosts with a method of notifying devices on the network that they
wish to leave the group.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
OL-29890-01 25
Configuring IGMP
IGMP Versions
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