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MST General Overview
VLANs into separate instances. See
for more information.
•
Map VLANs to MSTI – All existing VLANs are mapped to the default CIST instance 0.
Associating a VLAN to an MSTI specifies which Spanning Tree instance determines 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
page 5-47
and
"Sample MSTI Configuration" on page
MST Interoperability and Migration
Connecting an MSTP switch to a non-MSTP flat mode switch is supported. Since the Common and Inter-
nal Spanning Tree (CIST) controls the flat mode instance on both switches, STP or RSTP can remain
active on the non-MSTP switch within the network topology.
An MSTP switch is part of a Multiple Spanning Tree (MST) Region, which appears as a single, flat mode
instance to the non-MSTP switch. The port that connects the MSTP switch to the non-MSTP switch is
referred to as a boundary port. When a boundary port detects an STP (802.1D) or RSTP (802.1w) BPDU,
it responds with the appropriate protocol BPDU to provide interoperability between the two switches. This
interoperability also serves to indicate the edge of the MST region.
Interoperability between MSTP switches and per-VLAN mode switches is not recommended. The per-
VLAN mode is a proprietary implementation that creates a separate Spanning Tree instance for each
VLAN configured on the switch. The MSTP implementation is in compliance with the IEEE standard and
is only supported on flat mode switches.
Tagged BPDUs transmitted from a per-VLAN switch are ignored by a flat mode switch. This can cause a
network loop to go undetected. Although it is not recommended, you can also connect a per-VLAN switch
to a flat mode switch temporarily until migration to MSTP is complete. When a per-VLAN switch is
connected to a flat mode switch, configure only a fixed, untagged connection between VLAN 1 on both
switches.
Migrating from Flat Mode STP/RSTP to Flat Mode MSTP
Migrating an STP/RSTP flat mode switch to MSTP is relatively transparent. When STP or RSTP is the
active protocol, the Common and Internal Spanning Tree (CIST) controls the flat mode instance. If on the
same switch the protocol is changed to MSTP, the CIST still controls the flat mode instance.
Note the following when converting a flat mode STP/RSTP switch to MSTP:
•
Making a backup copy of the switch boot.cfg file before changing the protocol to MSTP is highly
recommended. Having a backup copy makes it easier to revert to the non-MSTP configuration if
necessary. Once MSTP is active, commands are written in their explicit form and not compatible with
previous releases of Spanning Tree.
•
When converting multiple switches, change the protocol to MSTP first on every switch before starting
to configure Multiple Spanning Tree Instances (MSTI).
•
Once the protocol is changed, MSTP features are available for configuration. Multiple Spanning Tree
Instances (MSTI) are now configurable for defining data paths for VLAN traffic. See
Works" on page 5-12
page 5-18
"What is a Multiple Spanning Tree Instance (MSTI)" on page 5-15
for more information.
OmniSwitch AOS Release 7 Network Configuration Guide
Configuring Spanning Tree Parameters
"Sample MST Region Configuration" on
5-49.
"How MSTP
March 2011
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