From: Brian (signal@xxxxxxxxxx)
Date: Thu Aug 22 2002 - 11:19:10 GMT-3
On Thu, 22 Aug 2002, Mingzhou Nie wrote:
> 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.
thats correct. If a hub is used, then you will just see some
sluggishness, due to the host having to figure out which traffic is valid
and which is not, and dropping the invalid traffic. The MAC (hardware)
will accept the packets of course, but the software must work to determine
its invalid packets.
>
> --- 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
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:48:33 GMT-3