===== STP and Flex Link =====
  * STP Phases
    * 802.1D
      * Blocking
      * Listening
      * Learning
      * Forwarding
  * Flex Links
    * (do on fa0/23) switchport backup int fa0/24
      * Switchport fa0/24 goes into a standby state in an active/standby design
      * Interface remains after failover (no preempt), unless fa0/24 loses connection