Multicast application and address

[sarahr202]

Hi everybody.

Let suppose we have a video server and some hosts. we want to implement multicasting, So a packet sent by video server is received by all hosts.

Let assume we choose the multicast group as 224.7.7.7.

My book says all the hosts should be running multicast applications configured with 224.7.7.7 . This makes complete sense. But then the book also says multicast application running on video server should also be configured with same multicast address i.e 224.7.7.7

But why? Traffic is flowing from video server to many hosts so why do we need multicast address on video server after all video server is not receiving multicast traffic. Rather it is sending traffic to many hosts.

I appreciate your help.

Thanks.

[Steve Fuller · Accepted solution]

Hi,

As you say it's correct that receivers have to join the multicast group that the source is sending to. This is achieved with the receiver sending an IGMP join to its first hop router and is how the receiver signals to that first hop router that it wishes to receive traffic destined to a specific group.

It is not correct that the source has to send an IGMP join before it can send to the group. The source can simply send traffic, which its first hop router will then forward in the case there are already registered receivers, or drop in the case there are no registered receivers.

Regards