Jon, thank you!
Even after the rewording I think it was your reference to only needing PIM on a single device but for multiple L3 devices you would want a multicast routing protocol.
I can still only read that as meaning PIM isn't a multicast routing protocol ie. you don't need a multicast routing protocol on a single device.
If Jon reads what I wrote to mean that, others might too.
I didn't mean to imply PIM isn't a multicast routing protocol or it's only needed on a single device.
As PIM is a multicast routing protocol, it also fulfills the condition of working for multiple L3 devices supporting multicast routing.
However, I did mean to say you may not need a multicast routing protocol on a single device.
My statement about PIM on a single Cisco device was based on my belief that you can multicast between a device's interfaces using PIM (DM interface statements), alone.
I suspect this extraordinary statement is what's causing the confusion, plus my belief might be wrong.
Privately, and recently, Jon has told me he tried interface PIM alone, with an emulator, and it didn't work until he also added ip multicast-routing. This is what one would expect, and is contrary to what I believed. However, Jon doesn't fully trust results on an emulator, although it's likely the emulator is correct.
Also privately, and recently, I asked Peter Paluch about this, because he seems more research oriented (i.e. more likely to do something like this in a lab, which you wouldn't do in normal practice). He too, thinks you must have the interface PIM statements, and expects you'll need the ip multicast-routing statement too, but he is not 100% positive on the latter. He said he might be able to lab it up. If he does, I'll post his results.
However, even if it does work, it's not something you would really want to do in practice. So too it was probably my mistake to even mention it, especially as I didn't emphasis my uncertainty.
