Website design By BotEap.comAs a CCNP and CCNA candidate, you may be tempted to skip or simply explore the many details of the Spanning Tree Protocol. After all, you learned all of that in your CCNA studies, right? That’s right, but it never hurts to check out STP for a switch exam! Also, many of us think of all four STP port states, but officially, there is a fifth!

Website design By BotEap.comDisabled is not generally considered to be an STP port state, but Cisco officially considers it to be an STP state. A disabled port is one that is administratively closed.

Website design By BotEap.comOnce the port is opened, the port will enter the blocking state. As its name implies, the port can’t do much in this state: no frame forwarding, no frame receiving, and therefore no MAC address learning. About the only thing this port can do is accept BPDUs from neighboring switches.

Website design By BotEap.comA port will then go from blocking mode to listening mode. The obvious question is “hear what?” Listening for BPDUs, and this port can now also send BPDUs. The port is not yet able to forward or receive data frames.

Website design By BotEap.comWhen the port goes from listening mode to learning mode, it is preparing to send and receive frames. In learning mode, the port begins learning MAC addresses in preparation for adding them to its MAC address table.

Website design By BotEap.comFinally, a port can go into forwarding mode. This allows a port to forward and receive data frames, send and receive BPDUs, and put MAC addresses in its MAC table.

Website design By BotEap.comTo view the STP mode of a particular interface, use the show spanning-tree interface command.

Website design By BotEap.comSW1#show rapid spanning tree interface 0/11

Website design By BotEap.comVlan Role Sts Cost Prior.Nbr Type

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Website design By BotEap.comVLAN0001 Design FWD 19 128.11 P2p

Website design By BotEap.comTo see these states in action, shut down a port on your CCNA/CCNP home lab and continuously run the show spanning interface command. Once you see this in action on real Cisco equipment, you will have no problem with the BCMSN exam questions. Just don’t practice this or any other Cisco command on a production network!