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

From: ZC Wong (zcwong_at_acm.org)
Date: 03/05/05 

Date: Sat, 05 Mar 2005 17:59:13 +0000

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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