Alcatel-Lucent 6850-48 ネットワークガイド

Spanning Tree Operating Modes

Configuring Spanning Tree Parameters

page 13-12

OmniSwitch AOS Release 6 Network Configuration Guide

The switch can operate in one of two Spanning Tree modes: flat and 1x1. Both modes apply to the entire 
switch and determine whether a single Spanning Tree instance is applied across multiple VLANs (flat 
mode) or a single instance is applied to each VLAN (1x1 mode). By default, a switch is running in the 1x1 
mode when it is first turned on. 

Use the 

 command to select the flat or 1x1 Spanning Tree mode.The switch operates in one 

mode or the other, however, it is not necessary to reboot the switch when changing modes. To determine 
which mode the switch is operating in, use the 

 command. For more informa-

tion about this command, see the OmniSwitch CLI Reference Guide.

Before selecting the flat Spanning Tree mode, consider the following:

• If STP (802.1D) is the active protocol, then there is one Spanning Tree instance for the entire switch; 

port states are determined across VLANs. If MSTP (802.1s) is the active protocol, then multiple 
instances up to a total of 17 are allowed. Port states, however, are still determined across VLANs.

• Multiple connections between switches are considered redundant paths even if they are associated with 

different VLANs. 

• Spanning Tree parameters are configured for the single flat mode instance. For example, if Spanning 

Tree is disabled on VLAN 1, then it is disabled for all VLANs. Disabling STP on any other VLAN, 
however, only exclude ports associated with that VLAN from the Spanning Tree Algorithm.

• Fixed (untagged) and 802.1Q tagged ports are supported in each VLAN. BPDU, however, are always 

untagged.

• When the Spanning Tree mode is changed from 1x1 to flat, ports still retain their VLAN associations 

but are now part of a single Spanning Tree instance that spans across all VLANs. As a result, a path 
that was forwarding traffic in the 1x1 mode may transition to a blocking state after the mode is changed 
to flat.

To change the Spanning Tree operating mode to flat, enter the following command:

-> bridge mode flat

The following diagram shows a flat mode switch with STP (802.1D) as the active protocol. All ports, 
regardless of their default VLAN configuration or tagged VLAN assignments, are considered part of one 
Spanning Tree instance. To see an example of a flat mode switch with MSTP (802.1s) as the active proto-
col, see