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Managing Multicast in an Internetwork

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Leaving Process

Withdrawal from a group is not initiated by the host as one would imagine.
The router hosts a timer that is reset every time a response is received from
a host on the subnet. The timer runs for 3 minutes, which is equivalent to 3
Membership Query cycles (every 60 seconds). If the timer expires and no
response is received from the hosts on the interface, the router disables mul-
ticast forwarding on that interface.

Internet Group Management Protocol Version 2 (IGMPv2)

As with any software revision, things are made better. Defined by RFC 2236,
Internet Group Management Protocol version 2 (IGMPv2) provides the
same functionality as version 1 did, but with a few enhancements:

The Leave process in version 2 was included to avoid long time-outs
that are experienced in version 1.

There are two Query forms, General and Group.

Network traffic is less bursty due to new timing mechanisms.

In the following sections, these enhancements will be discussed.

IGMPv2 Processes

It is important to be aware of issues when both versions of IGMP are present
on the network. Version 2 provides backward compatibility with version 1,
but the functionality of version 2 is lost when it's operating with version 1
devices. A version 2 host has to use version 1 frame formats when talking
with a version 1 router. The same applies when a version 2 router tries to
communicate with a version 1 host; it must use the version 1 format.

General and Group-Specific Query Processes

One enhancement that was made to IGMPv2 processes was the creation of
a new query type. The Membership Query, as it was called in IGMPv1, was
renamed General Queries, and the new type is Group-Specific Query. The
new query type is used to query a specific multicast group (kind of obvious
from the name). The overall procedure is the same as it is in IGMPv1.

When multiple IGMPv1 routers existed on the same segment, a multicast

routing protocol made the decision as to which of all the multicast routers
would perform the Membership Queries. Now, the decision is made using a

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