[LINK] Congestion (was Re: NBN to cost 24 times South Korea's faster network, says research body)

Richard Chirgwin rchirgwin at ozemail.com.au
Sun Feb 20 16:54:54 AEDT 2011

George, I'll pick this up, since it was my poor memory that gave this to 
IP multicast instead of Ethernet multicast.

Simple answer - one happens at layer 2, the other at layer 3.

Longer simplified answer.

Ethernet multicast is based on MAC addresses - so the same incoming 
Ethernet frame gets switched to multiple MAC addresses.

IP multicast works in routers - the same TCP/IP packet gets routed to 
multiple IP addresses.

It used to be that Ethernet (switching) would handle the same thing 
faster than IP (routing), but I guess that gap is narrowed somewhat!

In the case of the NBN - still sticking to a basic explanation that 
others can expand upon if they wish - it's because it keeps the roles 
clearly delineated. NBN Co is not supposed to route traffic; that's the 
job of the retail ISPs.

My question: is there a reason that multicast is necessary for NBN Co? 
Couldn't the retail ISPs simply use IP multicast themselves?


On 20/02/11 3:50 PM, George Bray wrote:
> On Mon, Feb 14, 2011 at 10:39 AM, Paul Brooks
> <pbrooks-link at layer10.com.au>  wrote:
>> Note also that as far as the NBN is concerned, 'multicast' is ethernet multicast, not
>> IP multicast (although the two layers might need to be connected), and the multicast
>> capability might well be unidirectional - from the PoI towards the user, not from
>> user-to-user.
>> Paul
> Paul, what's the difference between ethernet multicast and IP multicast?
> George
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