Spanning Tree Operating Modes; Using Flat Spanning Tree Mode - Alcatel OmniSwitch 6624 Network Configuration Manual

Spanning Tree Operating Modes

Spanning Tree Operating Modes
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 STP instance is applied across all VLANs, or a single STP instance
is applied to each VLAN. By default, a switch is running in the 1x1 mode when it is first turned on.
Use the bridge mode
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, see the OmniSwitch CLI Reference Guide.

Using Flat Spanning Tree Mode

Before selecting the flat Spanning Tree mode, consider the following:
There is one STP instance for the entire switch; port states are determined across VLANs.
•
Multiple connections between switches are considered redundant paths even if they are configured in
•
different VLANs.
STP parameters are configured for VLAN 1. For example, if STP is disabled on VLAN 1, then it is
•
disabled for all VLANs. Disabling STP on any other VLAN does not exclude 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 switch running in the flat STP mode. All ports, regardless of their default
VLAN configuration or tagged VLAN assignments, are considered part of one Spanning Tree instance.
page 5-8
command to select the flat or 1x1 Spanning Tree mode.The switch operates in one
show spantree
OmniSwitch 6624/6648 Network Configuration Guide
Configuring Spanning Tree Parameters
command. For more information about this
April 2004