Re: Thursday Follies [7:73323]
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
Re: Thursday Follies [7:73323]
""Ken Diliberto"" wrote in message news:[EMAIL PROTECTED] > Wait!! I know: > > 1 - Turn off the ports to the help desk so they will call you (you > can't call them because you can only use tools on the router or switch). > You can then have them change the fixed IP address on the old machine > since it's the one causing the conflict. :-) > > 2 - That depends on how aggressive the machine is. It may ask if the > address is in use or it might just tell everyone else the address is in > use. > > > You *did* say to have fun. :-) reminds me of a similar happening in a netwrok I used to manage. We started having problems with a particular user who traveled regularly, and when he returned to the office, his laptop acted up. he kept getting this dup ip address error. We were all on DHCP, but you know how Windows 95 was about that. In any case, the dup ip error box pops up on both computers, as you know. so I sent out an e-mail to everyone, asking them to call the help desk and report if they saw this dup ip box on their screen. nothing happened. few days later, after our wandering user return from another trip, I happened to be walking around in a certain part of the office. Low and behold, there on the screen of one of the broker's pc's was the dup ip dialogue box we had asked people to report. I pointed it out to the user, and asked why he didn't report it. he said "oh - I read your e-mail, but I didn't think this was what you were talking about" users. especially if they are brokers ;-> oh yeah - the solution was to eliminate that particular IP from the DHCP scope. for some reason the Win NT server we were using for DHCP would not play nicely with that particvular address. Once we made the change the problem did not occur again. one of those %^$%&$%%@ things, I guess. > > >>> "John Neiberger" 07/31/03 03:29PM > >>> > [snip] > > 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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=73328&t=73323 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Thursday Follies [7:73323]
Wait!! I know: 1 - Turn off the ports to the help desk so they will call you (you can't call them because you can only use tools on the router or switch). You can then have them change the fixed IP address on the old machine since it's the one causing the conflict. :-) 2 - That depends on how aggressive the machine is. It may ask if the address is in use or it might just tell everyone else the address is in use. You *did* say to have fun. :-) >>> "John Neiberger" 07/31/03 03:29PM >>> [snip] 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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=73327&t=73323 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Thursday Follies [7:73323]
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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=73323&t=73323 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
