Spanning Tree Operating Modes; Using Flat Spanning Tree Mode - Alcatel-Lucent OmniSwitch 9900 Series Network Configuration Manual

Configuring Spanning Tree Parameters

Spanning Tree Operating Modes

The switch can operate in one of two Spanning Tree modes: flat and per-VLAN. 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 (per-VLAN mode). A switch runs on the default
mode when it is first turned on.
Use the spantree mode
operates in one mode or the other, however, it is not necessary to reboot the switch when changing modes.

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 per-VLAN 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 per-VLAN mode can 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:
-> spantree 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
protocol, see "MST General Overview" on page
OmniSwitch AOS Release 8 Network Configuration Guide
command to select the Flat or Per-VLAN Spanning Tree mode.The switch
6-12.
Spanning Tree Operating Modes
December 2017 page 6-20