Multicast Design

Multicast Design
If the router interface is connected to a hub or a switch that doesn’t understand multicasting,
when the router forwards the multicast, the stream acts like a broadcast. In other words, every
device gets a copy. In IGMPv1, the router would keep forwarding the multicast stream out to
the hub, which forwards it to every connected client. Multicast routers work well because they
can forward a broadcast from one router to the next, something that doesn’t happen with true
broadcasts. The problem is that clients on a multicast segment get the stream whether they
want it or not.
This type of scenario is fine when the CEO wants to give a speech to every desktop, but
what about video that is only for a specific division, department, or business unit? If the
packets need to go to five different locations, and after you get past the routers all you have
are switches, everyone will receive the multicast stream. This doesn’t reduce bandwidth
utilization!
So far, corporate multicasting with IGMP, either version, works well at the router level. Too bad
most clients aren’t connected directly to router ports. Because IGMP is essentially nothing
more than intelligent broadcast propagation, Cisco created something that would enable
switches to participate as well: CGMP.