Cisco CCNP / BCMSN Exam Tutorial: The Four (Or Five) STP Port States
As a CCNP candidate and a CCNA, you may be tempted to skip or just browse the many details of Spanning Tree Protocol. After all, you learned all of that in your CCNA studies, right? That’s right, but it never hurts to review STP for a switching exam! Besides, many of us think of the four STP port states – but officially, there’s a fifth one!
Disabled isn’t generally thought of as an STP port state, 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...
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