Allied Telesis Rapier Switch - Manual (Page 49)

Layer 2 Switching 49
Rapier Switch Software Release 2.2.1
C613-02013-00 Rev A
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 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.
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.