Multicast Internet Route table.

John Kristoff jtk at
Tue Sep 2 16:30:44 UTC 2014

On Tue, 02 Sep 2014 08:43:16 -0700
Octavio Alvarez <alvarezp at> 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

> 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.


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