On Mon, 4 Sep 2000, Alan B. Pearce wrote:

> Whichever way it occurs, I still do not recall anybody mentioning what happens
> if you have multiple copies of the same MAC address on the same network. My
> memories of watching a network sniffer was that everything was related to the
> MAC address in terms of where the source and destination of the message were
> concerned. It is now a long time ago since I did this, so the memory may be
> getting hazy on this.

At Layer 2, systems do indeed use the MAC address.  Having the same MAC in
two places will have some undesired effects, the severity of which will
depend on the traffic.  If the two devices bearing the same MAC have
different IP addresses AND it's a shared media (hub or coax) environment,
I'm not sure you'd see a problem other than a little extra CPU load.  In a
switched network you'll see packet loss, as the "real" MAC holder may not
receive all packets that are sent to the "imposter's" switch port.  I
think the severity of the problems on a switched network will largely
depend on how the switch manufacturer deals with MAC learning.

In a switched network, we've seen switches hit near 100% CPU utilization
and drop horrendous numbers of packets while trying to keep up with the
same MAC address moving from port to port from the switch's point of view.
This was with about 70Mbit/sec of traffic on a 100M switch.  Effective
throughput was extremely low and connections timed out by the thousands.
it was really quite a mess, and it really sort of resembled a bridge loop.

But this is quickly moving in an [OT] direction, I think.

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
                -- Isaac Asimov

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