>>>> John Neiberger 7/31/03 5:02:31 PM >>> >Here's an interesting troubleshooting issue for you to chew on. There is a >fairly simple solution that may or may not be obvious at first. Heck, I >might have missed an even simpler solution. This is for the >beginner-to-intermediate level people. All you advanced people please >refrain from giving away the answer too quickly. :-) Here's the scenario: > >Imagine a location with a decent sized flat LAN where all hosts are supposed >to get their IP addressing information via DHCP. Well, DHCP was implemented >relatively recently and there is an old PC that was never converted to DHCP >that is now trying to connect to the network. It has a hard-coded IP address >that conflicts with one already in use, and the conflict causes the PC to >disconnect from the network when it detects that its IP address is being >used. > >1. Using tools available only on the router or switch, how do you find out >exactly which IP address is causing the conflict? > >2. Hint: how might a device determine if its own IP address is in use? > >Have fun! >John
Okay, here's my solution to this issue. On the router, use 'debug arp'. When a device comes online it will send an ARP request for its own IP address to make sure it isn't in use. If a device is already using that IP address it will respond. In the case I saw I was not able to see unicast responses. However, the second device subsequently sent out an ARP request for its own IP address immediately after seeing someone else trying to nab its address. So, in the output of debug arp you should see two consecutive or nearly-consecutive ARP requests for the same IP address coming from different MAC addresses. If it were necessary you could use that information to find the device in the MAC address tables on the switch. Perhaps a Friday follies is to follow later today.... John Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=73357&t=73323 -------------------------------------------------- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com
