Alcatel-Lucent 6850-48 ネットワークガイド

Spanning Tree Overview

Configuring Spanning Tree Parameters

page 13-8

OmniSwitch AOS Release 6 Network Configuration Guide

Once the Spanning Tree is calculated, there is only one root bridge, one designated bridge for each LAN, 
and one root port on each bridge (except for the root bridge). Data travels back and forth between bridges 
over forwarding port connections that form the best, non-redundant path to the root. The active topology 
ensures that network loops do not exist.

Switches send layer 2 frames, referred to as Configuration Bridge Protocol Data Units (BPDU), to relay 
information to other switches. The information in these BPDU is used to calculate and reconfigure the 
Spanning Tree topology. A Configuration BPDU contains the following information that pertains to the 
bridge transmitting the BPDU:

The sending and receiving of Configuration BPDU between switches participating in the bridged network 
constitute the root bridge election; the best path to the root is determined and then advertised to the rest of 
the network. BPDU provide enough information for the STP software running on each switch to deter-
mine the following:

• Which bridge will serve as the root bridge.

• The shortest path between each bridge and the root bridge.

• Which bridge will serve as the designated bridge for the LAN.

• Which port on each bridge will serve as the root port.

• The port state (forwarding or discarding) for each bridge port based on the role the port will play in the 

active Spanning Tree topology.

The following events trigger the transmitting and/or processing of BPDU in order to discover and main-
tain the Spanning Tree topology:

• When a bridge first comes up, it assumes it is the root and starts transmitting Configuration BPDU on 

all its active ports advertising its own bridge ID as the root bridge ID.

The Bridge ID for the bridge that this bridge believes is the root.

Root Path Cost The sum of the Path Costs that lead from the root bridge to this bridge port.

The Path Cost is a configurable parameter value. The IEEE 802.1D standard specifies a 
default value that is based on port speed. See 

 for more information.

An eight-byte hex value that identifies this bridge within the Spanning Tree. The first 
two bytes contain a configurable priority value and the remaining six bytes contain a 
bridge MAC address. See 

 for more 

information.

Each switch chassis is assigned a dedicated base MAC address. This is the MAC 
address that is combined with the priority value to provide a unique Bridge ID for the 
switch. For more information about the base MAC address, see the appropriate Hard-
ware Users Guide for the switch.

A 16-bit hex value that identifies the bridge port that transmitted this BPDU. The first 4 
bits contain a configurable priority value and the remaining 12 bits contain the physical 
switch port number. See 

 for more informa-

tion.