13. Rapid Spanning Tree Protocol (RSTP)MN700004 Rev 01 123• a Backup Port - if the Bridge is the Designated Bridge for the attached LANOR OTHERWISE• an Alternate Port.Alternate and Backup PortsAn Alternate Port offers a path in the direction of the Root Bridge alternate to that providedby the Bridge’s Root Port.A Backup Port acts as a backup for the path provided by a Designated Port in the direction ofthe leaves of the Spanning Tree. Backup Ports exist only where a given Bridge has two ormore connections to a given LAN, Therefore, backup ports (and the Designated Ports thatthey back up) can exist only where two ports are connected together in loopback by a point topoint link, or where the Bridge has two or more connections to a shared media LAN segment.The distinction between the Alternate and Backup Port Roles was introduced in RSTP inorder to describe the possibility of the rapid transition of an Alternate port to forwarding if theRoot Port fails.Point-To-Point LinksSome of the rapid state transitions that are possible within RSTP depend on whether the Portconcerned can be connected to only one other Bridge (i.e., it is served by a point-to-pointLAN segment), or to two or more Bridges (i.e., it is served by a shared medium LANsegment).Rapid transition of a Designated Port to Forwarding is possible only if the LAN segmentassociated with the Port is point-to-point, or if the port is defined to be an edge Port.Otherwise, the transition of a Designated Port from Discarding to Learning and from learningto Forwarding occurs with a delay of Forward Delay.Changing Port StatesThe Port States are controlled by a state machine, designed to maximize connectivity withoutintroducing temporary data loops in the network. The state machine attempts to transitionRoot Ports and Designated Ports to the Forwarding Port State, and Alternate Ports andBackup Ports to the Discarding Port State, as rapidly as possible.Transitions to the Discarding Port State can be simply effected without the risk of data loops.Transition of a Port to the Forwarding Port State needs to be consistent with the Port Rolesassigned to other Ports in the region of the network.A Bridge knows that the transition to the Forwarding Port State can be made if:1. The Port Role has been Root Port or Designated Port long enough FOR:spanning Tree information supporting this role assignment to have reached all Bridges inthe network,AND FOR
