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MST Configuration Overview Using 802.1s Multiple Spanning Tree
page 6-10 OmniSwitch 6600 Family Network Configuration Guide April 2006
MST Configuration Overview
The following general steps are required to set up a Multiple Spanning Tree (MST) configuration:
• Select the flat Spanning Tree mode. By default, each switch runs in the 1x1 mode. MSTP is only
supported on a flat mode switch. See “Understanding Spanning Tree Modes” on page 6-11 for more
information.
• Select the 802.1s protocol. By default, each switch uses the 802.1D protocol. Selecting 802.1s acti-
vates the Multiple Spanning Tree Protocol (MSTP). See “How MSTP Works” on page 6-4 for more
information.
• Configure an MST region name and revision level. Switches that share the same MST region name,
revision level, and VLAN to Multiple Spanning Tree Instance (MSTI) mapping belong to the same
MST region. See “What is a Multiple Spanning Tree Region” on page 6-8 for more information.
• Configure MSTIs. By default, every switch has a Common and Internal Spanning Tree (CIST)
instance 0, which is also referred to as MSTI 0. Configuration of additional MSTI is required to
segment switch VLANs into separate instances. See “What is a Multiple Spanning Tree Instance
(MSTI)” on page 6-7 for more information.
• Map VLANs to MSTI. By default, all existing VLANs are mapped to the CIST instance 0. Associat-
ing a VLAN to an MSTI specifies which Spanning Tree instance will determine the best data path for
traffic carried on the VLAN. In addition, the VLAN-to-MSTI mapping is also one of three MST
configuration attributes used to determine that the switch belongs to a particular MST region.
For a tutorial on setting up an example MST configuration, see “Quick Steps for Configuring an MST
Region” on page 6-14 and “Quick Steps for Configuring MSTIs” on page 6-16.
Using Spanning Tree Configuration Commands
The Alcatel implementation of the 802.1s Multiple Spanning Tree Protocol introduces the concept of
implicit and explicit CLI commands for Spanning Tree configuration and verification. Explicit commands
contain one of the following keywords that specifies the type of Spanning Tree instance to modify:
• cist–command applies to the Common and Internal Spanning Tree instance.
• msti–command applies to the specified 802.1s Multiple Spanning Tree Instance.
• 1x1–command applies to the specified VLAN instance.
Explicit commands allow the configuration of a particular Spanning Tree instance independent of which
mode and/or protocol is currently active on the switch. The configuration, however, does not go active
until the switch is changed to the appropriate mode. For example, if the switch is running in the 1x1 mode,
the following explicit commands changes the MSTI 3 priority to 12288:
-> bridge msti 3 priority 12288
Even though the above command is accepted in the 1x1 mode, the new priority value does not take effect
until the switch mode is changed to flat mode.
Note that explicit commands using the cist and msti keywords are required to define an MSTP (802.1s)
configuration. Implicit commands are only allowed for defining STP or RSTP configurations.