Spanning Tree Protocol (802.1D)

STP is enabled by Default in Cisco Switches. STP constructs a loop-free shortest path network and allows for duplicate or redundant paths.


There is a separate STP in each VLAN


States of STP


Frame Processing


Blocked (max age)

(Discarding State for RSTP)

No frames forwarded, Receiving BPDUs

20 Sec


No frames forwarded, Listening frames, BPDUs processed (sent/recd) to determine paths and build topology

15 Sec


No frames forwarded, Learning addresses, builds bridging table

15 Sec


Frames forwarded. Sending/Receiving data



No frames forwarded, No BPDUs heard. Administratively down



Note:BPDUs sent out every 2 seconds by default.


Note: Rapid STP (802.1w) can bypass Listening and Learning states because it can skip the process on Edge Type shared

and Edge Type Point-to-point links.


Switches selected based on BID (Bridge ID), and the lowest will become the Root Bridge.

Bridge ID is made up of a Bridge Priority and the MAC address.


Priority can be changed with the following switch command

Switch(config)#spanning-tree vlan 1 priority <number> (number can be any value from 0-65535. Default is 32768)


Once converged, BPDUs are only sent out from Root Switch (except RSTP). If no BPDUs are heard when Max Age is reached

Then switch initiates negotiations to rebuild Spanning Tree. It does this by transitioning to the Listening State, and follows

the same process as the startup.


During the Election process, the root bridge, root ports and designated ports are elected. Root bridge is decided as above, by BID. Root ports and designated ports are selected based on cumulative path costs, based on the following table:


Total or cumulative path cost

4 Mbps


10 Mbps


16 Mbps


45 Mbps


100 Mbps

19 (previously 10)

155 Mbps


622 Mbps


1 Gbps

4 (previously 1)

10 Gbps

2 (previously 1)




STP prevents three problems in switch networks:

         Broadcast Storms

         Multiple Frame Transmissions

         Unstable MAC Databases




Switch show and debug commands

show spanning-tree

show run