STP (Spanning Tree Protocol) States
Ref: http://www.excitingip.com/1688/understanding-spanning-tree-protocols-stp-rstp-mstp/
Ref: http://www.ccnacertificationguide.com/STP_States.php
Ref: http://www.omnisecu.com/cisco-certified-network-associate-ccna/spanning-tree-port-states.htm