On Fri, Dec 2, 2011 at 12:14 PM, Lamar Owen <lo...@pari.edu> wrote:
>
>> My point is that every device on your network has to process every
>> broadcast packet. Maybe you have CPU overkill on all your computers,
>> but you might also have some dumb controllers too. And they have to
>> go out the wifi too.
>
> Ethernet multicast frames, depending on switch implementation, may go to
> every device as well. How the NIC responds to multicast ethernet frames is
> likely implementation-dependent, but, again, I don't have metrics on that,
> except for a failure mode on a couple of controller-type devices we
> experienced once.
NICs have to accept broadcasts and pass them up to CPU-type processes.
They may have efficient mechanisms to ignore multicasts they aren't
interested in (but yes, it is implementation - dependent).
> This was not broadcast traffic; it was multicast traffic, but the switches
> flooded those frames to every port in that VLAN anyway, and a controller that
> was not a member of that multicast group got flooded. Now, to be fair, the
> SitePlayer Telnet does Bonjour, and thus does respond to mDNS, but that is
> supposed to be on a different multicast group. Whether that was a factor or
> not I don't know; but I do know that the Davis Instruments ethernet devices
> we have, and that don't do mDNS, also went offline during the multicast event.
>
> So, no numerical metrics, but anecdotal evidence that multicast can be just
> as bad as broadcast to controllers with insufficient bandwidth or CPU power.
Yes, if you fill the media capacity, things start to break, and on a
point-to-point connection like 10BT there's not much difference
between the media and the endpoints.
> (And it pointed to the fact that those SitePlayer Telnet boxes really should
> have been on a different VLAN and thus in a different broadcast domain.......)
You might still flood the VLAN trunked connections between switches
where the media is shared - but in practice that would probably be a
higher capacity.
--
Les Mikesell
lesmikes...@gmail.com
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