duplicate

[Ibrahim Jamil]

Hi

when u receive duplicate ip address on the pc,pls be noted the ms DHCP handle ips to client

how to start troubleshoot this issue in order to fix it and find the out the root cuse for it

thanks

[Dan-Ciprian Cicioiu]

Hi ,

you might try finding the host with the same ip, looking after the mac-address of the rogue PC on the switch.

Long term solution will be : dhcp snooping + arp inspection.

Dan

[Ibrahim Jamil]

hi , can y clarify more , how to do it?

[Leo Laohoo]

When you get a duplicate IP address error message and your IP address is handled by DHCP the most common culprit is someone has configured a static IP address onto the second client.

How to troubleshoot?   Well, let's presume that your switch won't see anything so the duplicate IP address is seen by the client with the DHCP IP address.

If this is the case, then this is what I would normally do:

1.  Take note of the duplicate IP address;

2.  Disable the switch port of the client with DHCP IP address.  This leaves the cllient with static address on the network.  Ping that IP address and the client with the static IP address will respond.

3.  On a layer 3 device (router or a layer 3 switch) try to run the command "sh ip arp ".  You'll get a MAC address.  Note this down.

4.  On a switch, run the command "sh mac- address " or "sh mac address ".  The result will give you a switch port to start tracing.

5.  When you find the culprit, pound him/her on the head.

[Dan-Ciprian Cicioiu]

Shut down the port of the PC - the one that has the IP though DHCP.

Check the router's arp table ( show ip arp  x.x.x.x )  for the PC's arp , it should be the rogue PC - check the mac address with the one of the PC.

Then check the mac-address-table of the intern switches  ( show mac address-table | include h.h.h ) for the mac address that you got from the arp table of the router.

Then you should know the port. At this point you know what is the device that has the same IP.

Dan

[Ibrahim Jamil]

thank you