Spanning Tree Introduction: STP, RSTP, MSTP
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
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 (
Table 18-1
). The basic algorithm is similar for STP, RSTP and MSTP modes.
Table 18-1: Spanning tree process
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 (XEM
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 XEM or base-board.
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).