Configuring Spanning Tree Parameters MST General Overview
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-12
MST General Overview
The Multiple Spanning Tree (MST) feature allows for the mapping of one or more VLANs to a single
Spanning Tree instance, referred to as a Multiple Spanning Tree Instance (MSTI), when the switch is
running in the flat Spanning Tree mode. MST uses the Multiple Spanning Tree Algorithm and Protocol
(MSTP) to define the Spanning Tree path for each MSTI. In addition, MSTP provides the ability to group
switches into MST Regions. An MST Region appears as a single, flat Spanning Tree instance to switches
outside the region.
This section provides an overview of the MST feature that includes the following topics:
• “How MSTP Works” on page 6-12.
• “Comparing MSTP with STP and RSTP” on page 6-15.
• “What is a Multiple Spanning Tree Instance (MSTI)” on page 6-15.
• “What is a Multiple Spanning Tree Region” on page 6-16.
• “What is the Internal Spanning Tree (IST) Instance” on page 6-17.
• “What is the Common and Internal Spanning Tree Instance” on page 6-17.
• “MST Configuration Overview” on page 6-17.
How MSTP Works
MSTP, as defined in the IEEE 802.1Q 2005 standard, is an enhancement to the IEEE 802.1Q Common
Spanning Tree (CST). The CST is a single spanning tree that uses 802.1D (STP) or 802.1w (RSTP) to
provide a loop-free network topology.
The OmniSwitch flat spanning tree mode applies a single CST instance on a per switch basis. The per-
VLAN mode is an OmniSwitch proprietary implementation that applies a single spanning tree instance on
a per VLAN basis. MSTP is only supported in the flat mode and allows for the configuration of additional
Spanning Tree instances instead of just the one CST.
On an MSTP flat mode OmniSwitch, the CST is represented by the Common and Internal Spanning Tree
(CIST) instance 0 and exists on all switches. Up to 17 instances, including the CIST, are supported. Each
additional instance created is referred to as a Multiple Spanning Tree Instance (MSTI). An MSTI
represents a configurable association between a single Spanning Tree instance and a set of VLANs.
The following diagrams help to further explain how MSTP works by comparing how port states are
determined on per-VLAN STP/RSTP mode, flat mode STP/RSTP, and flat mode MSTP switches.
Note. Although MSTP provides the ability to define MSTIs while running in the flat mode, port state and
role computations are automatically calculated by the CST algorithm across all MSTIs. However, it is
possible to configure the priority and/or path cost of a port for a particular MSTI so that a port remains in a
forwarding state for an MSTI instance, even if it is blocked as a result of automatic CST computations for
other instances.
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