On Tue, 02 Sep 2014 08:43:16 -0700 Octavio Alvarez <alvarezp@alvarezp.ods.org> wrote:
No not all and even those that do often do not do so on the same gear, links and peers as their unicast forwarding.
Why would that be, are network devices not able to support multicast?
That was part of it, but there is also some benefit to separating it. When I ran networks with it, we essentially had it everywhere so I'm not the best person to explain the reasons others may have had. IP multicast can be difficult to troubleshoot and maintain, especially when it often runs for months on end without issue, until it doesn't. There may be practical scaling limitations and security issues. So isolation in part may be both a practical necessity and an operational safeguard.
I have never used interdomain multicast but I imagine the global m-routing table would quickly become large.
The routes wouldn't need to look much different than unicast, but when you have to start maintaining other state other than just route reachability, particularly for tracking participants in groups and sources, things can get unwieldy fast. The best thing I can say about IP multicast is that it nice experience to *have had*. Note, this is not to disparage decisions by those who choose to use IP multicast for certain circumstances, which is still in widespread use and successful such as with TV over cable networks, but those are largely isolated environments and configurations are generally set in such a way that much of the scaling, state and security issues are sufficiently dealt with. John