Isolating a mac address

luwhite · ‎08-09-2007

I have a rogue DHCP machine and have used Windows "dhcploc" tool - along with Ethereal - to track down both the ip address and mac-address of this rogue device. After obtaining both of these, I went to my switch (Cisco 2950) and ran "show mac-address-table" to find where the mac-address was physically plugged into the switch. The results do not show it to be on any FastEthernet port but rather on my Gigabit port. Even more oddly is, unlike all the other listings which show the ports as either FA0/1 or Gi0/1, this particular one is showing only "Gi0/". How do I find this device when it shows on my fiber port? I've traced everywhere I can think of. Thanks, Lu

Jon Marshall · ‎08-09-2007

Hi

What is on the other end of the fibre link ?

Jon

luwhite · ‎08-09-2007

an identical switch (2950). I forgot to mention that the ip address of this unknown device is a 192.168.168.x which is clearly not our internal addressing scheme either.

Jon Marshall · ‎08-09-2007

Have you tried "sh mac-address-table" on this other switch then ?

Jon

luwhite · ‎08-09-2007

yes, and it doesn't show this mac-address at all. Only the one switch does.

wochanda · ‎08-09-2007

Grab another PC and have it request an IP address right before you do this. It could be that nobody has requested an IP address in 5 minutes, so the mac-address entries have aged out of your switches.