Mst Overview - Cisco Nexus 9000 Series Configuration Manual

MST Overview

MST Overview
Note You must enable MST; Rapid PVST+ is the default spanning tree mode.
MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology
independent of other spanning tree instances. This architecture provides multiple forwarding paths for data
traffic, enables load balancing, and reduces the number of STP instances required to support a large number
of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding
path) does not affect other instances (forwarding paths).
MST provides rapid convergence through explicit handshaking because each MST instance uses the IEEE
802.1w standard, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and
designated ports to the forwarding state.
MAC address reduction is always enabled on the device. You cannot disable this feature.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
• Original 802.1D spanning tree
• Rapid per-VLAN spanning tree (Rapid PVST+)
• IEEE 802.1 was defined in the Rapid Spanning Tree Protocol (RSTP) and was incorporated into
Note
• IEEE 802.1 was defined in MST and was incorporated into IEEE 802.1Q.
MST Regions
To allow devices to participate in MST instances, you must consistently configure the devices with the same
MST configuration information.
A collection of interconnected devices that have the same MST configuration is an MST region. An MST
region is a linked group of MST bridges with the same MST configuration.
The MST configuration controls the MST region to which each device belongs. The configuration includes
the name of the region, the revision number, and the VLAN-to-MST instance assignment mapping.
A region can have one or multiple members with the same MST configuration. Each member must be capable
of processing 802.1w bridge protocol data units (BPDUs). There is no limit to the number of MST regions in
a network.
Each device can support up to 65 MST instances (MSTIs), including Instance 0, in a single MST region.
Instances are identified by any number in the range from 1 to 4094. The system reserves Instance 0 for a
special instance, which is the IST. You can assign a VLAN to only one MST instance at a time.
The MST region appears as a single bridge to adjacent MST regions and to other Rapid PVST+ regions and
802.1D spanning tree protocols.
Cisco Nexus 9000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
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IEEE 802.1D.
Configuring MST Using Cisco NX-OS