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New Member

%IP-4-DUPADDR: Duplicate address

Hi, I've been getting the follwoing message on my c2600 (version 13.3) router since last december (according to the log file).

005179: .Dec 28 04:35:08.652 AUS: %IP-4-DUPADDR: Duplicate address on

FastEthernet0/0, sourced by 0015.f987.941a

The symptom of the problem is that occassionally the network looses connactivity.

The mac addresses are both for Cisco devices. One is the router and i'm not sure what the other is possibly a firewall.

The Cisco site says that we can change the mac address of one of the offending items ?

Which would be the best one to change?

How can i find out what the other device is?

What if any issue arise with changing the mac address ?

Proably other questions that i should be asking as well, but i'll get to those later.

thanks in advance for any reponses.

  • Application Networking
Cisco Employee

Re: %IP-4-DUPADDR: Duplicate address

changing the mac-address will not help.

You have to change the ip address.

You first have to find out what/where is the other device.

For that you can follow the L2 path.

Go the switch to which the router is connected on fast0/0. If you don't know the switch, try 'sho cdp nei'.

Once on the switch, check the mac-address with the command 'show mac-address address x.x.x'. It should give the port.

Check what device is attached to this port.

If another switch, repeat the operation above until you find the end device.


New Member

Re: %IP-4-DUPADDR: Duplicate address

The only trouble with that suggest is that this router happens to be in another state.

I'll have to try and talk somebody though the trace. show cdp nei does not work for the second device, it must be turned off.

If it is a firewall and i change the IP address is that going to affect anything?

Does the router tables need to be updated etc? Does updating take time?

Cisco Employee

Re: %IP-4-DUPADDR: Duplicate address

If it was a firewall I think you would have much more trouble. It must instead be a router or a host.

I can't tell you what will be the impact of changing the ip on the other device until we know what kind of device it is.

For sure no propagation accross the network will be required.


New Member

Re: %IP-4-DUPADDR: Duplicate address

The issue here is some user in the network made a mistake and write his IP address as the gateway.

To solve this issue you must know who's this user by show arp | inc (the mac address that is shown) then the ip will appear.

show mac-address-table add (the mac address that is shown) and you will find the port that which user assigned his IP as the gateway(interface vlan ip)