Re: What's gonna happen if two clients in the same LAN have the same MAC address?

From: Roger Abell (mvpNOSpam_at_asu.edu)
Date: 03/05/05 

Date: Sat, 5 Mar 2005 12:02:42 -0700

Hub just makes one machine seeing the conflict more likely
as all packets route to all ports of hub, unlike switch.
Hang could be due to being in a syncronous state waiting
on response that does not happen, but that sounds like bad
software design. ? Perhaps bad packet (aimed for the other)
was accepted and threw handler into tight failure/retry state.
Keep in mind the you are dealing with things that run in
the kernel state and can preempt you from seeing activity
at user mode.
To see what actually happened you would need to have
info on the actual packet stream (capture) and debugger
hooked up so you could interrupt and examine the "hung"
box. 

-- 
Roger
"ZC Wong" <zcwong@acm.org> wrote in message
news:eWjoUyaIFHA.2736@TK2MSFTNGP09.phx.gbl...
> Roger Abell wrote:
> > The short answer is that things do not work correctly.
> >
> > Since packet flow in the local network is done using the
> > MAC addresses the switches/routers will be choosing
> > which route a packet takes based on the client it has last
> > heard from/believed (if there is a routing conflict between
> > the two machines, as would be so in fully switched setup).
> > That DHCP will have given out the same IP to the two is
> > a further complication, and also indicates the two will be
> > on the same network segment.  Both clients will be listening
> > for packets addressed to the MAC, and most likely one will
> > see them as fine while the other will not see the sequencing
> > as valid.  Things will happen like one client appearing to
> > work outgoing but not getting any response (like ping doing
> > a timeout when pinging the default gateway IP).
> > With both on the same segment the machines can notice one
> > another, even on a fully switched network due to broadcasts.
> > When the machines see there is a conflict, depending on version
> > of Windows, they give a popup and/or write an event log msg.
>
> Thanks for your reply.
>
> I've just tried in a switch based network, and my Windows Server 2003
> hangs after conflict, but I wonder what actually happened. I guess if
> there are two machines with the same MAC then the switch got to keep
> updating its port-MAC mapping table upon client's request? But how did
> the crash happen on my machine?
>
> By the way, will it be a problem of receiving extra packets if those
> machines are connected to the same hub instead of a switch?

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