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Cisco 2901 Duplicate Address Doesn't Exist

dburns865
Beginner
Beginner

I have been having some issues with our Cisco 2901 including this log I keep getting that says "%IP-4-DUPADDR: Duplicate address x.x.x.x on GigabitEthernet0/0.xxx, sourced by <firewall-mac-address>". This should be as simple as tracking down the interface on the firewall with the duplicate address, but it does not exist. I have done a 'show interfaces all' command and there is no duplicate address on the firewall. There has been a few other issues with the router lately that I am having a hard time figuring out, including a bunch of overflow errors despite there not being a lot of traffic and high CPU usage. Any idea what would cause this duplicate ip error when there isn't a duplicate IP? 

2 Replies 2

Seb Rupik
VIP Advisor VIP Advisor
VIP Advisor

HI there,

You may have a Layer 2 loop on the network segment hanging off that sub-interface. A broadcast packet originating from the router is being sent round this loop and forwarded back to the router.

 

Check your downstream switch topology.

 

cheers,

Seb.

Simon Darlington
Beginner
Beginner

Hi there

As the device in question is a firewall I wonder if the mac-address in question is a virtual mac-address, which is why you can't correlate this to a mac-address on a physical interface? This virtual mac-address can be shared between redundant firewalls, which may explain the log entry. In which case this would not strictly be a problem.

 

I would check how virtual mac-addresses are handled by your particular make and model as a first step.

 

Hope this helps. Please rate if it does.

 

Thanks

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