Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Identical MAC address

when I do a sh arp on my 2811 series Router I am getting mostly all the LAN IP addresses displaying identical MAC address and also that MAC address is showing conflict on with the Router fastethernet interface. The problem is that most of the PC in the LAN are unable to use Internet. All other services are working fine. Pls help to resolute.

Hall of Fame Super Silver

Re: Identical MAC address


I have seen somewhat similar symptoms of multiple devices seeming to have the same MAC address when the connection to them was through a firewall. Is that perhaps the case here?

It would help if you could tell us a bit more about the physical connectivity in this situation. What is the 2811 physically connected to (switch or whatever) and what are the PCs connected to?

It would also be helpful if you would post the configuration of the 2811.




Re: Identical MAC address

Is there NAT involved?

New Member

Re: Identical MAC address

Are you using HSRP?

The router's mac (without HSRP) will show up as many destinations, since it will answer for PCs that require proxy-arp. (proxy arp happens if the PC has an incorrectly configured mask, or for any reason sends out an arp request for something on its directly connected LAN, instead of sending the packet to its default gateway).

The router therefore can wind up representing many destinations if you're looking in the ARp table. But... no other address should share the router's macaddress if you're looking in any arp table.

Can you do a show arp from a PC or show mac-address-table from the router to show us what is being seen? also the IP address of the router in question would be helpful.


CreatePlease login to create content