For switches to participate in the same Multiple Spanning Tree Protocol (MSTP) region and achieve load balancing, they must share an identical MSTP configuration. A critical component of this configuration is the VLAN-to-instance mapping table. This table defines which VLANs belong to which Spanning Tree Instance (MSTI). By creating different instances and assigning different root bridges to each, traffic for different VLANs can be forced to take different paths. All switches in the region must have the exact same VLAN-to-instance mapping to ensure a consistent and loop-free topology for each instance, which is the fundamental requirement for load balancing.

A. Spanning-tree bpdu-guard setting: This is a port-level security feature to prevent rogue switches and is not a parameter for defining an MSTP region or enabling load balancing.

C. spanning-tree Cist mapping: The CIST (Common and Internal Spanning Tree, or Instance 0) is the default instance. The key to load balancing is the specific mapping of VLANs to multiple different instances (MSTI 1, MSTI 2, etc.), not just the CIST.

D. Spanning-tree root-guard setting: This is a port-level security feature to enforce the root bridge placement in the network; it does not need to be identical across switches for region formation.

1. ArubaOS-CX 10.12 Spanning Tree Protocol Guide

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"MSTP regions" section: "An MSTP region is a group of interconnected switches that have the same MSTP configuration. The MSTP configuration consists of the following attributes: ... VLAN-to-instance mapping table." The document further states

"To be in the same MSTP region

switches must have the same configuration for all of these attributes."

2. ArubaOS-CX 10.12 Spanning Tree Protocol Guide

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"Multiple Spanning Tree Protocol (MSTP)" section: "MSTP provides for load balancing of traffic for different VLANs by assigning different VLANs to different MSTP instances (MSTIs). Each MSTI can have a different root switch and a different topology." This directly links the concept of VLAN-to-instance mapping to the goal of load balancing.