On Fri, 14 May 2010 15:30:22 -0700, Darren New wrote:

> Jim Henderson wrote:
>> Depends on how the router implements forwarding for multicast - ISTR
>> that some routers can be configured to forward multicast traffic up to
>> a specified TTL, but what's forwarded is still multicast traffic.
> 
> Yes. And if you have a router with three ethernet connections plugged
> into it, it's going to duplicate each multicast packet onto each
> ethernet connection. Otherwise it isn't multicast.

Assuming the switch isn't multicast aware.  I wouldn't be surprised if 
some were these days (but I haven't looked at it recently).

> The whole multicast protocol is nothing more than telling each router on
> the path which outputs it needs to duplicate packets on to, so you
> *don't* get multicast going onto network segments where nobody is
> listening for them.

Right.

> Of course when it's duplicated it's still a multicast packet. That's
> defined by the source address, and on any given segment that supports
> broadcast it is treated as a broadcast packet. It's still point-to-point
> between routers. I.e., it's still point to point if you treat "all the
> people listening to the same broadcast address on the same network
> segment" as one point of the multicast transmission.

It's more like multiple people listening to a radio station - the data is 
only transmitted once per subnet.  The difference between, for example, 
using unicast to push an image down to 15 workstations on a subnet and 
using multicast to push an image down to 15 workstations on a subnet is a 
significant reduction in overall network traffic.

Jim

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