Bertram
As a CCNP candidate and a CCNA, you may possibly be tempted to skip or just browse the many details of Spanning Tree Protocol. Immediately after all, you learned all of that in your CCNA studies, appropriate? That is proper, but it never ever hurts to assessment STP for a switching exam! In addition to, many of us believe of the 4 STP port states - but officially, there is a fifth one!
Disabled is not typically believed of as an STP port state, but Cisco does officially take into account this to be an STP state. A disabled port is 1 that is administratively shut down.
As soon as the port is opened, the port will go into blocking state. As the name implies, the port can not do a lot in this state - no frame forwarding, no frame receiving, and therefore no understanding 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 question is "listening for what?" Listening for BPDUs - and this port can now send BPDUs as nicely. The port still can't forward or receive information frames.
When the port goes from listening mode to studying mode, it is finding ready to send and receive frames. In studying mode, the port starts to find out MAC addresses in preparation for adding them to its MAC address table.
Lastly, a port can go into forwarding mode. This allows a port to forward and get data frames, send and acquire BPDUs, and spot MAC addresses in its MAC table.
To see the STP mode of a offered interface, use the show spanning-tree interface command.
SW1#show spanning-tree interface quickly /11
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Vlan Function Sts Expense Prio.Nbr Sort
---- --- --------- -------- ----------
VLAN0001 Desg FWD 19 128.11 P2p
To see these states in action, shut a port down in your CCNA / CCNP property lab and continually run the show spanning interface command. Once you see this in action on true Cisco gear, you are going to have no difficulty with BCMSN exam inquiries. Just do not practice this or any other Cisco command on a production network!
