Allied Telesis C613-02013-00 manual Spanning Tree Protocol STP, Spanning Tree Port States

Layer 2 Switching		49
	Table 8: Parameters in the output of the SHOW SWITCH QOS command
	Parameter	Meaning
	Priority level	The priority level of the frame.
	QOS egress queue	The Quality Of Service egress queue that frames with this
		priority level join.

Spanning Tree Protocol (STP)

The Spanning Tree Protocol (STP) makes it possible to automatically disable redundant paths in a network to avoid loops, and enable them when a fault in the network means they are needed to keep traffic flowing. A sequence of LANs and switches may be connected together in an arbitrary physical topology resulting in more than one path between any two switches. If a loop exists, frames transmitted onto the extended LAN would circulate around the loop indefinitely, decreasing the performance of the extended LAN. On the other hand, multiple paths through the extended LAN provide the opportunity for redundancy and backup in the event of a bridge experiencing a fatal error condition.

The spanning tree algorithm ensures that the extended LAN contains no loops and that all LANs are connected by:

■Detecting the presence of loops and automatically computing a logical loop-free portion of the topology, called a spanning tree. The topology is dynamically pruned to a spanning tree by declaring the ports on a switch redundant, and placing the ports into a ‘Blocking’ state.

■Automatically recovering from a switch failure that would partition the extended LAN by reconfiguring the spanning tree to use redundant paths, if available.

Spanning Tree Port States

Each port can be in one of five Spanning Tree states, and one of two switch states. The Spanning Tree states (Table 9) affect the behaviour of ports whose switch state is enabled.

Table 9: Spanning tree port states

State	Meaning
DISABLED	STP operations are disabled on the port. The port can still
	switch if its switch state is enabled.
LISTENING	The port is enabled for receiving frames only.
LEARNING	The port is enabled for receiving frames only, and the
	Learning Process can add new source address information
	to the Forwarding Database.
FORWARDING	The normal state for a switch port. The Forwarding Process
	and the Spanning Tree entity are enabled for transmit and
	receive operations on the port.
BLOCKING	The Spanning Tree entity has disabled the Forwarding
	process for transmit and receive operations on the port, but
	the Spanning Tree entity itself remains enabled for transmit
	and receive operations on the port.

Rapier Switch Software Release 2.2.1 C613-02013-00 Rev A