Derose

As a CCNP candidate and a CCNA, you may be tempted to skip or just browse the numerous information of Spanning Tree Protocol. Following all, you learned all of that in your CCNA studies, proper? That is correct, but it in no way hurts to overview STP for a switching exam! Apart from, many of us feel of the four STP port states - but officially, there is a fifth 1!

Disabled isn't usually believed of as an STP port state, but Cisco does officially think about 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 not do much in this state - no frame forwarding, no frame receiving, and consequently no finding out of MAC addresses. About the only factor this port can do is accept BPDUs from neighboring switches.

A port will then go from blocking mode into listening mode. The obvious query is "listening for what?" Listening for BPDUs - and this port can now send BPDUs as effectively. The port nevertheless can't forward or get data frames.

like i said

When the port goes from listening mode to understanding mode, it is getting ready to send and get frames. In understanding mode, the port starts to discover MAC addresses in preparation for adding them to its MAC address table.

Ultimately, a port can go into forwarding mode. This enables a port to forward and acquire data frames, send and obtain 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.

SW1#show spanning-tree interface quick /11

Vlan Part Sts Price Prio.Nbr Type

---

---- --- --------- -------- ----------

VLAN0001 Desg FWD 19 128.11 P2p

To see these states in action, shut a port down in your CCNA / CCNP residence lab and continually run the show spanning interface command. Once you see this in action on genuine Cisco equipment, you'll have no difficulty with BCMSN exam inquiries. Just don't practice this or any other Cisco command on a production network!