Allied Telesis Rapier Switch - Manual (Page 53)

Layer 2 Switching 53
Software Release 2.5.1
C613-02025-00 REV B
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 Modes
STP can run in STANDARD mode, or RAPID mode. Rapid mode allows for
rapid configuration of the spanning tree. The Rapid Spanning Tree Protocol
(RSTP) is specified in IEEE 802.1w.
A spanning tree running in standard mode can take up to one minute to rebuild
after a topology or configuration change. The Rapid Spanning Tree algorithm
provides for a more rapid recovery of connectivity following the failure of a
bridge, bridge port, or a LAN.
For information about RSTP see the Rapid Mode Spanning Tree Types section,
Switch chapter in the Rapier Switch Software Reference.
Spanning Tree and Rapid Spanning Tree Port States
If STP is running in STANDARD mode, then each port can be in one of five
Spanning Tree states, and one of two switch states. If STP is running in RAPID
mode, then each port can be in one of four states. The state of a switch port is
taken into account by STP. To be involved in STP negotiations, STP must be
enabled on the switch, the port must be enabled on the switch, and enabled for
the STP it belongs to.
The Spanning Tree states (see Table 16) affect the behaviour of ports whose
switch state is enabled.
Table 16: 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.