From: Mingzhou Nie (mnie@xxxxxxxxx)
Date: Thu Aug 22 2002 - 10:59:15 GMT-3
A switch will listen to incoming packet to learn source mac. At t1,
host1 on port 2/1 is sending traffic to host3, switch will add a cam
entry. at t2, host2 on port 2/2 is sending traffic, switch sees same
mac from different port, it will modify the cam entry. at t3, host3
reply to host1, but switch will forward traffic to port 2/2 to host2.
So the answer is it will cause problems if switch is used. It won't be
a problem is hub is used.
--- Michael Snyder <msnyder@revolutioncomputer.com> wrote:
> The only time I've seen multiple cam entries going to different ports
> is
> with multicast mac addresses.
>
> I think multiple unicast mac addresses would be a sign of a layer two
> loop to a switch.
>
> Set span enable does something like what you describe, but that's a
> special case, where the switch is echoing the traffic on two ports.
>
> About your question about if the workstations would be fuctional?
> Maybe, but the number of problems, for example what if both machines
> where using ftp at the same time, or both using the port 80 web
> services
> at the same time. Any personal firewall is going to see this as a
> spoofing attack.
>
>
> I stand by my statement that two of the same unicast mac addresses in
> the same subnet is a very bad thing.
>
>
> On the a side note, if you wish to drive a network administrator
> nuts,
> randomly set the mac address's (some nic cards allow mac's to be set)
> on
> random computers in the office to the same value. I only know a few
> administrators that could figure it out.
>
> -----Original Message-----
> From: Brian [mailto:signal@shreve.net]
> Sent: Thursday, August 22, 2002 8:03 AM
> To: msnyder@revolutioncomputer.com
> Cc: ccielab@groupstudy.com
> Subject: RE: Challenge Question
>
> On Thu, 22 Aug 2002, Michael Snyder wrote:
>
> > The two ip address with one mac address isn't a problem. You can
> go
> > into the ip advanced settings on a w2k box, and setup as many
> addresses
> > as you like. As long as the machine responds to the arp ip lookup,
> it
> > works.
> >
> > Two of the same mac addresses on the same subnet would be a bad
> thing.
> > Might work if you are using a hub instead of a switch, but both
> machines
> > would get each others traffic.
>
> Are you 100% convinced that a switch would not install 2 CAM entries
> and
>
> just send data to both? As far as both machines receiving eachothers
>
> traffic, do you believe this would create a problem where the
> function
> is
> lost, or just slow performance.........think about how that traffic
> would
> be handled.
>
> Brian
>
>
> > > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> Behalf
> Of
> > Brian
> > Sent: Thursday, August 22, 2002 6:34 AM
> > To: ccielab@groupstudy.com
> > Subject: Challenge Question
> >
> > I know this group likes these challenge questions, so I have one
> for
> you
> >
> > and hopefully it has not been put to this group recently.
> >
> > You have two hosts each with identical MAC addresses on an ethernet
> LAN.
> > They also have identical IP addresses. Why or why not would this
> be a
>
> > problem for the client communicating (assuming each of the dupe
> machines
> >
> > doesnt need to communicate with eachother only to other hosts on
> the
> LAN
> > and through the gateway)?
> >
> > Ok, similar to above, same MAC addresses but different IP
> addresses.
> > Why
> > or why not would this create communications on the LAN or through
> the
> > gateway?
> >
> > good luck!
> >
> > Brian
> >
> >
> >
> > -----------------------------------------------
> > Brian Feeny, CCIE #8036 e: signal@shreve.net
> > Network Engineer p: 318.222.2638x109
> > ShreveNet Inc. f: 318.221.6612
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