Alcatel-Lucent 6850-48 Guida Di Riferimento

Distributed Spanning Tree Commands

page 15-4

OmniSwitch CLI Reference Guide

Selects a flat Spanning Tree or 1x1 Spanning Tree operating mode for the switch. These modes are exclu-
sive; however, it is not necessary to reboot the switch when changing modes.

bridge mode {flat | 1x1} 

One Spanning Tree instance per switch. 

One Spanning Tree instance for each VLAN configured on a switch.

By default, the bridge mode for the switch is set to 1x1 Spanning Tree.

OmniSwitch 6400, 6850, 6855, 9000, 9000E

• The Multiple Spanning Tree Protocol (MSTP), as defined in the IEEE 802.1Q 2005 standard, is only 

supported on switches operating in the flat Spanning Tree mode.

• If standard STP or RSTP is used when the switch is running in the flat mode, a single STP instance is 

applied across all VLANs. For example, if a port belonging to VLAN 10 and a port belonging to 
VLAN 20 both connect to the same switch, then STP will block one of these ports.

• If MSTP is used when the switch is running in the flat mode, a single STP instance is applied to each 

Multiple Spanning Tree Instance (MSTI). Each MSTI represents a set of VLANs.

• Flat Spanning Tree mode supports fixed (untagged) and 802.1Q tagged ports in each VLAN. However, 

Bridge Protocol Data Units (BPDUs) are always untagged.

• If 1x1 mode is selected, a single Spanning Tree instance is enabled for each VLAN configured on the 

switch. For example, if there are five VLANs configured on the switch, then there are five separate 
Spanning Tree instances. In essence, a VLAN is a virtual bridge in that it will have its own bridge ID 
and configurable STP parameters, such as protocol, priority, hello time, max age, and forward delay. 

• When operating in 1x1 mode, 802.1Q tagged ports participate in an 802.1Q Spanning Tree instance 

that allows the Spanning Tree to extend across tagged VLANs. As a result, a tagged port may partici-
pate in more than one Spanning Tree instance; one for each VLAN that the port carries.

• If a VLAN contains both fixed and tagged ports and the switch is operating in 1x1 Spanning Tree 

mode, then a hybrid of the two Spanning Tree instances (single and 802.1Q) is applied. If a VLAN 
appears as a tag on a port, then the BPDU for that VLAN are also tagged. However, if a VLAN appears 
as the configured default VLAN for the port, then BPDU are not tagged and the single Spanning Tree 
instance applies.

• Regardless of which mode the switch is running in, it is possible to administratively disable the Span-

ning Tree status for an individual VLAN (see 

that active ports associated with such a VLAN are excluded from any Spanning Tree calculations and 
will remain in a forwarding state.