responses, it will build a list of all the multicast groups for that interface. Any other router on the same subnet
will also receive the responses and will have the same information.
Note A host does not have to wait for a host membership query to send out a membership report. It could send
an unsolicited membership report, a process sometimes mistakenly referred to as sending out an IGMP
join to the router.
To minimize the traffic, IGMP utilizes a report suppression mechanism. The host starts a countdown with a
random value between zero and the maximum response interval for each multicast group to which it belongs.
When the value reaches zero, the host sends a membership report. All members of the same multicast group
receive this message and reset their own countdown values. As long as the router receives a membership
response, it will not send out a host membership query.
To leave a multicast group, a host simply stops sending membership responses. If no group members send
member responses, the router begins its own timer (usually one minute). It will then send a membership query
and reset its timer. If the router sends a membership query three times and does not get a reply, it will stop
sending the multicast group’s traffic out that interface.
The time interval from when the last host leaves the multicast group until the router stops transmitting traffic
can create network problems. Assume that a user is trying to find a channel (multicast group) on a multimedia
application. As the user surfs the different channels to locate the one he wants, he will join multiple multicast
groups. If he goes through six channels before finding the one he wants, the router will transmit all six
multicast groups’ traffic until the timer runs out. This process may not affect the user much, but if the traffic is
coming across a WAN link, the unnecessary high−bandwidth multimedia traffic will bog down the link.
IGMPv2 was developed with this situation in mind.
IGMPv2
IGMPv2 functions the same as version 1, with the following enhancements:
Querier election process—Allows the routers on the local subnet to elect the designated IGMP
querier utilizing IGMP rather than using an upper−layer protocol, as in version 1. The router with the
lowest IP address is elected and is responsible for sending out the multicast query for that subnet.
•
Maximum response time—Allows the router to set an upper limit for the value the host will use to
determine when to send a membership report. This limit enables you to fine−tune your IGMP traffic.
•
Multicast group−specific query message—An enhancement to the original all−host membership
query. Rather that being sent to everyone, a multicast query will be sent only to the specific multicast
group.
•
Leave group message—The biggest improvement in IGMPv2. In version 2, a host joins a multicast
group the same way as in version 1. However, the process of leaving is completely different. Instead
of just keeping quiet, the host sends a leave group message to the all−routers (224.0.0.2) multicast
group. When a router receives this message, it sends out a multicast group−specific query. (The router
must send this query because it only keeps a list of multicast groups associated with each interface. It
doesn’t know if any other multicast group members exist.) When another host on the same subnet
receives the query, it responds with a membership report. The router will then maintain that multicast
group’s association to that interface. Just as in version 1, other members of the same multicast group
will see the reply and not respond to the query. If the host is the last member of the multicast group
and no other host responds to the query, the router will wait a last member query interval (default
value one second) and send out another group−specific query. If it doesn’t receive any reply to this
second query, the router will stop transmitting multicast traffic out that interface. This process results
in a much faster response time than in version 1 when a host leaves the multicast group.
•
Tip By default, Cisco routers utilize version 2. If any host on the subnet does not support version 2, you
must change the router to version 1.
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