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Enabling/Disabling Spanning Tree for a VLAN Configuring VLANs

page 5-10 OmniSwitch 6800/6850/9000 Network Configuration Guide June 2006

Enabling/Disabling Spanning Tree for a VLAN

When a VLAN is created, an 802.1D standard Spanning Tree Algorithm and Protocol (STP) instance is

enabled for the VLAN by default. On the OmniSwitch 6800 and 6850, an 802.1w Rapid Spanning Tree

Algorithm and Protocol (RSTP) instance is enabled for the VLAN by default.

The spanning tree operating mode set for the switch determines how VLAN ports are evaluated to identify

redundant data paths. If the Spanning Tree switch operating mode is set to flat, then VLAN port connec-

tions are checked against other VLAN port connections for redundant data paths. Note that the single flat

mode STP instance is referred to as instance 1 or the CIST (Common and Internal Spanning Tree)

instance, depending on which STP protocol is active.

In the flat mode, if STP instance 1 or the CIST instance is disabled, then it is disabled for all configured

VLANs. However, disabling STP on an individual VLAN will exclude only that VLAN’s ports from the

flat STP algorithm.

If the Spanning Tree operating mode is set to 1x1, there is a single Spanning Tree instance for each VLAN

broadcast domain. Enabling or disabling STP on a VLAN in this mode will include or exclude the VLAN

from the 1x1 STP algorithm.

The vlan stp command is used to enable/disable a Spanning Tree instance for an existing VLAN. In the

following examples, Spanning Tree is disabled on VLAN 255 and enabled on VLAN 755:

-> vlan 255 stp disable

-> vlan 755 stp enable

Note the following when using the vlan stp command. For more information about the vlan stp command,

see the OmniSwitch CLI Reference Guide:

• If the VLAN ID specified with this command is that of a VLAN that does not exist, the VLAN is auto-

matically created.

• This command configures the VLAN STP status for both the 1x1 and flat Spanning Tree modes. Using

the 1x1 or flat parameter with this command, configures the STP status only for the mode specified by

the parameter.

• Up to 253 Spanning Tree instances per switch are supported in the 1x1 Spanning Tree mode. Since

each VLAN with Spanning Tree enabled uses one of these instances, only 253 VLANs can have an

active Spanning Tree instance at any given time.

• To create more than 253 VLANs on a switch running in the 1x1 Spanning Tree mode, use the vlan stp

disable, vlan 1x1 stp disable, or vlan flat stp disable form of this command to create a VLAN with

Spanning Tree disabled.

STP does not become operationally active on a VLAN unless the VLAN is operationally active, which

occurs when at least one active port is assigned to the VLAN. Also, STP is enabled/disabled on individual

ports. So even if STP is enabled for the VLAN, a port assigned to that VLAN must also have STP enabled.

See Chapter 6, “Configuring Spanning Tree Parameters.”

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