28.1.3 Spanning Tree and Rapid Spanning Tree

There are two types of PVST+ and Single Spanning Tree: IEEE 802.1D Spanning Tree Protocols and IEEE 802.1w rapid Spanning Tree Protocols. These are called PVST+ and Rapid PVST+, and STP and Rapid STP.

When a communication path changes, the topology calculation for the Spanning Tree Protocol immediately puts the port in Blocking status (communication is not possible), switches to multiple statuses, and then puts it in Forwarding status (communication is possible). Because IEEE 802.1D Spanning Tree Protocols perform this status transition by a timer, a set time is required until communication is possible. IEEE 802.1w rapid Spanning Tree Protocols omit this timer-based waiting time for status transitions to perform high-speed status transitions, minimizing the time for which communication stops due to topology changes.

Note that because Multiple Spanning Tree is standardized under IEEE 802.1s, the status transition times is the same as for IEEE 802.1w. The following table describes the status transitions for each protocol, and their corresponding required times.

Table 28-3 State transitions of PVST+ and STP (single spanning tree)

Status	Status overview	Transition to the next status
Disable	Status in which a port cannot be used. This transitions to Blocking as soon as the port becomes available.	-
Blocking	Status in which communication is not possible.In this status, MAC address learning is not performed. This is the status after link-up or of ports after topology stabilization and blocking.	20 seconds (variable) or until BPDU reception
Listening	Status in which communication is not possible.In this status, MAC address learning is not performed. This is the duration until the topology stabilizes before the corresponding port is learned.	15 seconds (variable)
Learning	Status in which communication is not possible. In this case, however, MAC address learning is performed. This is the duration for which MAC address learning is performed before the corresponding port transitions to Forwarding.	15 seconds (variable)
Forwarding	Status in which communication is possible. In this case, the topology is stable.	-

Table 28-4 Rapid PVST+ and Rapid STP (Single Spanning Tree) State Transition

Status	Status overview	Transition to the next status
Disable	Status in which a port cannot be used. This transitions to Discarding as soon as the port becomes available.	-
Discarding	Status in which communication is not possible.In this status, MAC address learning is not performed. This is the duration until the topology stabilizes before the corresponding port is learned.	Omitted or 15 seconds (variable)
Learning	Status in which communication is not possible. In this case, however, MAC address learning is performed. This is the duration for which MAC address learning is performed before the corresponding port transitions to Forwarding.	Omitted or 15 seconds (variable)
Forwarding	Status in which communication is possible. In this case, the topology is stable.	-

With Rapid PVST+ and Rapid STP, the Discarding and Learning statuses are skipped by BPDU reception from the partner switch. This enables high-speed topology changes.

When using a rapid Spanning Tree Protocol, set it according to the conditions described below. If these conditions are not satisfied, discarding and learning might not be skipped, and high-speed status transitions might not be performed.

• Build the whole topology with the same protocols (Rapid PVST + or Rapid STP). (See Section 28.3.2, "PVST of Access-Port+" for the interconnection between Rapid PVST+ and Rapid STP.)

• Point-to-Point connections are used between switches running for the Spanning Tree Protocol.

• PortFast is set on ports not connected to switches running for the Spanning Tree Protocol.