Mst Regions; Mst Bpdus - Cisco Nexus 3000 series Configuration Manual

MST Regions

MST provides rapid convergence through explicit handshaking as 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 while you are using MST. 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 802.1s defined MST and was incorporated into IEEE 802.1Q.
MST Regions
To allow switches to participate in MST instances, you must consistently configure the switches with the
same MST configuration information.
A collection of interconnected switches 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 switch belongs. The configuration includes
the name of the region, the revision number, and the MST VLAN-to-instance assignment map.
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 region can support up to 65 MST instances (MSTIs). 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.
We recommend that you do not partition the network into a large number of regions.
Note

MST BPDUs

Each region has only one MST BPDU, and that BPDU carries an M-record for each MSTI within the region
(see the following figure). Only the IST sends BPDUs for the MST region; all M-records are encapsulated in
Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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IEEE 802.1w defined the Rapid Spanning Tree Protocol (RSTP) and was incorporated into IEEE 802.1D.
Configuring Multiple Spanning Tree