When I started studying for the CCNA, I remember memorizing the STP port states. All four of them. Then years later I heard about the "five STP port states".
Five? How did Cisco invent a 5th STP port state?
Well, they really didn't. There is a fifth port state that I personally don't really think of as a real port state,but the important thing is that Cisco does! To help you with your studies, let's review the STP port states - all four or five of them!
Disabled isn't generally thought of as an STP port state by many, but Cisco does officially consider this to be an STP state. A disabled port is one that is administratively shut down.
Once the port is opened, the port will go into blocking state. As the name implies, the port can't do much in this state - no frame forwarding, no frame receiving, and therefore no learning of MAC addresses. About the only thing this port can do is accept BPDUs from neighboring switches.
A port will then go from blocking mode into listening mode. The obvious question is "listening for what?" Listening for BPDUs - and this port can now send BPDUs as well. The port still can't forward or receive data frames.
When the port goes from listening mode to learning mode, it's getting ready to send and receive frames. In learning mode, the port begins to learn MAC addresses in preparation for adding them to its MAC address table.
Finally, a port can go into forwarding mode. This allows a port to forward and receive data frames, send and receive BPDUs, and place MAC addresses in its MAC table.
To see the STP mode of a given interface, use the show spanning-tree interface command.
To see these states in action, shut a port down in your CCNA / CCNP home lab and continually run the show spanning interface command. Once you see this in action on real Cisco equipment, you'll have no problem with your exam questions. Just don't practice this or any other Cisco command on a production network!
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