Overview Of Spanning Trees - Allied Telesis SwitchBlade x3100 Series Manual

Introduction
MSTP is defined in IEEE standard 802.1Q-2005. The protocol builds on, and remains compatible with, the pre-
vious IEEE standards defining STP and RSTP. (MSTP is provided in release 14.2.)

4.5.2 Overview of Spanning Trees

A Spanning Tree instance is a (named) logical representation of the underlying data structures and control
mechanisms that provide a simple, fully-connected active network topology for a set of bridges and the LANs
that connect them in a network.
4.5.3 Spanning Tree Protocol (STP and RSTP)
The Spanning Tree Protocol (STP) makes it possible to automatically disable redundant paths in a network to
avoid network loops, and to re-enable them when it is necessary to maintain connectivity in the event of a fault
in the network (such as the failure of a link or a switch).
The spanning tree algorithm prunes redundant paths from the topology (i.e. marking paths as unavailable so
frames are not transmitted over those paths). The resulting loop-free topology set of switches and active paths
is called the logical spanning tree.
A logical spanning tree has the following elements:
Each switch in the extended LAN has a unique bridge ID. This is a combination of the a switch's priority
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component (a value assigned by default or via manual configuration) and its the switch's MAC address.
The switch with the numerically lowest bridge ID is considered the root bridge of the logical spanning tree.
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Each port on a switch has a unique port ID. This is a combination of the port's priority component (a value
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assigned by default or via manual configuration) and an internally assigned, unique numeric location identifier
local to the bridge.
Each port connecting a switch to a LAN has an associated path cost. This is a value assigned by switch
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software as a default based on port speed, or via manual user configuration, that provides an indication of
the latency or resource consumption that would be encountered if a frame were to be transmitted on that
port.
The root path cost for a particular path from a port, a LAN, or switch to the root bridge is the sum of the
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port path costs incurred if a frame were to be transmitted on that path to the root bridge.
The root port of a switch is the port on the switch with the lowest root path cost. If two or more ports on
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a switch have the same root path cost, the root port is the port with the numerically lowest port ID.
The designated bridge of a LAN is the switch on the LAN with the lowest root path cost. If two or more
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switches on the LAN have the root path cost, the designated bridge is the switch with the lowest bridge ID.
A designated port of a switch is a port that connects a LAN to its designated bridge.
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For the SBx3112, RSTP is the default STP setting.
Note:
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Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
Overview of Spanning Trees