Subscribing and Maintaining Groups

Subscribing and Maintaining Groups
For multicast traffic to reach a host, that host must be running an application that sends a request
to a multicast-enabled router informing the router that it wishes to receive data belonging to the
specified multicast group. If this request were never to take place, the router wouldn’t be aware
that the host was waiting for data from the specified group.
Consider this overview: A multicast-enabled router receives all group advertisements and
routes. It listens on all interfaces, waiting for a request from a host to forward multicast group
traffic. After a host on an interface makes a request to become a member of a group, the interface
activates the requested group on itself and only on itself. While the host is a member, multicast
data is forwarded to that interface, and any host subscribed to the group receives the data.
That was a simple overview. Now let’s look at how this is accomplished in more detail. We
start by discussing five major host subscription protocols:

IGMPv1

IGMPv2

IGMPv3

CGMP

IGMP Snooping
The differences among them will become apparent as we get further into the discussion.