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New Member

Same IP address, different device

Has anyone seen this problem ? We swapped over a network printer the other day, and the new device could not be seen on the network until we changed its IP address - even if we were directly telnetted into the switch it was attached to !

Maybe this is some kind of security setting on the 3560 ? I tried flushing the ARP table on the switch, but no joy - I couln't do a switch power cycle as we were in the middle of the day, and it's a production network. Double and triple checked the address, mask and gateway of the network printer, all fine.

I would love to know if anyone else has seen this type of behaviour ..

3 REPLIES

Re: Same IP address, different device

Hi Friend,

From this only thing I can think of is that when you have changed the hardware i.e new printer the mac entry for that hardware must be different.

Now the old printer mac address is already binded with that ip address and when you might have added the new printer with same ip address it will try to bind the same ip address with new mac address and there could have been a problem.

If you are doing all your routing on 3560 then clearing the arp entries on 3560 would have resolved the problem but if you are using 3560 just a layer 2 switch then clearing arp entry will not resolve anyting cause ARP entry will be there on layer 3 device which is doing your routing part.

ARP entry remain for approx 4 hours if it is in idle state.

HTH, if yes please rate the post.

Ankur

New Member

Re: Same IP address, different device

This makes a lot of sense, as the 3560 is used as a L2 device - we run a 6506 and 2x3750 stacked as dual L3 core routers. But it doesn't make sense in another aspect - the 3560 is on a different subnet to the core routers so surely the problem would then move up to L3, as the core would hold only the IP address, not the mac address ?

(confused!)

Re: Same IP address, different device

Hi Friend,

Lets take this way your cat6k is layer 3 and it has a binding for your old printer with its mac address and the ip address. Any one from other subnet will reach cat6k and cat6k will say to reach your printer your cat6k will give the mac address which he had in its arp entry.

Now you removed the old printer but arp entry still exist for that ip address with old mac address. And now you attached the new printer which will be having different mac address but you assigned the same ip address. Your layer 2 switch 3560 will add the new mac address but new arp will not be there so any one again from other subnet will try to reach new printer with same ip will reach cat6k and cat6k will check its arp entry which is already there with old mac address and will see that to reach that ip it has the mac address but that mac address is not at all present now cause you have removed the old printer so there will be a confusion and no one will be able to reach the new printer.

If you would have clear the arp entry on cat6k then cat6k would have again broadcast for arp entry for that ip address and would have learned the new mac.

HTH, if yes please rate the post.

Ankur

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