Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) User Manual
Page 471
Spanning Tree Introduction: STP, RSTP, and MSTP
Software Reference for x310 Series Switches
C613-50046-01 REV A
AlliedWare Plus
TM
Operating System - Version 5.4.4C
18.3
The logical tree computed by the spanning tree algorithm has the following properties:
■
A single bridge is selected to become the spanning tree’s unique root bridge. This is
the device that advertises the lowest Bridge ID. Each bridge is uniquely identified by
its Bridge ID, which comprises the bridge’s root priority (a spanning tree parameter)
followed by its MAC address.
■
Each bridge or LAN in the tree, except the root bridge, has a unique parent, known as
the designated bridge. Each LAN has a single bridge, called the designated bridge, that
connects it to the next LAN on the path towards the root bridge.
■
Each port connecting a bridge to a LAN has an associated cost, called the root path
cost. This is the sum of the costs for each path between the particular bridge port and
the root bridge. The designated bridge for a LAN is the one that advertises the lowest
root path cost. If two bridges on the same LAN have the same lowest root path cost,
then the switch with the lowest bridge ID becomes the designated bridge.
The spanning tree computation is a continuous, distributed process to establish and
maintain a spanning tree (
). The basic algorithm is similar for STP, RSTP and
MSTP modes.
1.
The whole three part port number (D.M.P) is used to find the lowest port number, where
D is the device number within a stack (1 for a non stacked device), M is the module
number within the device (note that 0 is used for all base-board connected ports), and P is
the number of the port within the base-board.
Table 18-1: Spanning tree process
The spanning tree
algorithm ...
By ...
Selects a root bridge
It selects as the root bridge for the spanning tree the
device with the (numerically) lowest bridge identifier
(that is, the device with lowest root bridge priority
value, or if they have the same priority, the bridge
with the lowest MAC address).
Selects root ports
On each device, it selects the root port according to:
■
the port with the lowest path cost to the root
bridge
■
the port connected to the bridge with the lowest
root identifier
■
MSTP and RSTP only: the port with the lowest port
priority value
■
the port with the lowest port number
1
Blocks alternate ports
In order to prevent loops, it blocks alternate ports
(Discarding state) that provide higher cost paths to
the root bridge.
Blocks backup ports
Where a second port connects one switch back to
itself, it blocks the backup port that has the highest
path cost or port number.
Selects designated ports
All other ports that are not disabled are selected as
designated ports and are eventually made active
(Forwarding state).
Maintains the spanning tree
If a switch or port fails, the spanning tree configures a
new active topology, changing some port states, to
reestablish connectivity and block loops. Depending
on where the failure occurs, the changes may be
widespread (e.g., if the root bridge fails), or local (e.g.,
if a designated port fails).