Cisco Systems N3KC3048TP1GE manual MST Regions, MST BPDUs, MST BPDU with M-Records for MSTIs

Configuring Multiple Spanning Tree

MST Regions

Note You must enable MST; Rapid PVST+ is the default spanning tree mode.

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.

Note We recommend that you do not partition the network into a large number of regions.

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 that one BPDU that the IST sends. Because the MST BPDU carries information for all instances, the number of BPDUs that need to be processed to support MSTIs is significantly reduced.

Figure 14: MST BPDU with M-Records for MSTIs

	Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
90	OL-26590-01