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. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

duplicate mac address


Can somebody tell me how a switch will react if it keeps seeing the SAME MAC address on multiple switch ports ? Will it forward the packets for the MAC to all switch port, clear oldest entries for the MAC address ?? Clear whole CAM table ?


New Member

Re: duplicate mac address

This is something I actually tried out a long time ago.

And it really depends on which switch you are using.

At the time I only had access to lower end equipment from netgear and the alike.

Most switches tended to forward the packet to the port which first came up when you did a "show mac-address-table" or equivalent.

The mac address table stayed the same during mac expiry and updates.


Re: duplicate mac address

As mentioned, the exact behavior will vary. The same MAC address on multiple ports is generally considered an error condition, however, and should be treated as such. Cisco switches will log errors when this occurs, and from what I've seen will just keep updating the CAM entry in question until the issue is resolved.

I think forwarding the packets to multiple ports in this situation would probably be the worst possible outcome. Hopefully no switches do this.