User:Msal5342

'STP port states'

Ports in STP may exist in one of the following states; ports may transit from one state to another according to network changes:

"Blocking" state Don't forward frames, only receive BPDUs. If the port in "Blocking state" didn't receive any BPDU during "Maximum age time = 20 sec.", the port will enter to "listening state".

"Listening" state Don't forward frames; only receive BPDUs to determine the port role in STP. The port will remain in "listening state" for "forward delay time = 15 sec.", then the port will enter to "Learning state".

"Learning" state Don't forward frames; receive BPDUs to learn all network paths. The port will remain in "Learning state" for "forward delay time = 15 sec.", then it will enter to "forwarding state.

"Forwarding" state The port forwards and receives all data frames. The port in STP may stay only in "blocking state" or "forwarding state" "Listening state" and "forwarding state" are transient states between "blocking state" and "forwarding state".

STP convergence time: The time consumed until all ports are in either a "blocking state" or "forwarding state"

As you can observe, the time consumed for a port to transit from "Blocking state" to "forwarding state" is a long time. (max age time 20sec. + forward delay time 15 sec. + forward delay time 15 sec.). So, the port may need about 50 sec. to transit from "Blocking state" to "forwarding state" which is a very long time. This problem may affect network stability.

source: http://www.aboutccna.com/spanning-tree-protocol-stp/stp-port-states.html]