Membership Query Process

Membership Query Process
One important process is the
IGMP Query process
, which is kindred to a keepalive procedure.
Because the router needs to keep tabs on which multicast groups need to remain active, or be
made active or inactive, it sends a Membership Query out each interface. The query is directed
to the reserved address of 224.0.0.1, to which all multicast hosts will answer.
After the request is received, the hosts report back with their group subscription information.
After a specific group has been reported to the router, subsequent reports for the same group
coming from different hosts are suppressed. This is done because only one host on a subnet/
VLAN needs to request membership for the router to activate that group on the interface. Once
active on the router interface, any host on that segment wanting to receive data for that specific
group will receive it. Figure 19.8 depicts how this process works.
You can follow the numbers indicated in this figure. First, the query to 224.0.0.1 is sent, and
subsequently, the hosts begin to report back. The first host to respond (#2a) is Host B, requesting
data for the multicast group 224.2.127.254. Host D responds next (#3a) with a request for
the group 224.2.168.242. The next host to reply is Host A (#4a). However, because the report
from Host D was already multicast to the 224.2.168.242 group, Host A heard the report and
suppressed its own report to the group.