Alcatel-Lucent OmniSwitch 6800 Series - Spanning Tree Operating Modes; Using Flat Spanning Tree Mode

Spanning Tree Operating Modes Configuring Spanning Tree Parameters
page 7-12 OmniSwitch 6800/6850/9000 Network Configuration Guide March 2008
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 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 bridge mode 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 bridge rrstp ring vlan-tag command. For more informa-
tion about this command, see the OmniSwitch CLI Reference Guide.
Using Flat Spanning Tree Mode
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 Chapter 2, “Using 802.1Q 2005 Multiple Spanning Tree.”