Also remember that if you use port-security with the maximum option on your interfaces the mac-address-table entries learned will be static and consequently not listed with the dynamic option!

Ex:
 switchport port-security maximum 3

Thanks for your observation, but I don’t believe it’s an actual issue, but rather a logical behavior. If you connect a device in a cisco switch port, the interface will come up showing connected. This is Layer 1 in OSI stack, the physical connection. If there is zero communication from device, then the switch doesn’t have a trigger for an ARP request and it will not learn any mac-address of that port.As soon as there is any active communication (like DHCP request) then the mac will appear on that particular port.Of course there are some strange situation when there is something misconfigured and even with active communication there is no mac on Cisco switch port, but this fall into another category.

Some times the interface shows status of connected but with the command above you realize there is no MAC address associated to the switch port at issue.

Then the possible cause for that has been already discussed in your article: Speed/Duplex mismatch or a defective network adapter on the attached device.

I am new to your website, I think it is great. Thanks for sharing your knowledge.

Keep up the great work!

Have a good day,

David M.