In a Fortinet Multi-Chassis Link Aggregation Group (MCLAG) configuration, the peer switches are designed to operate as a single logical switch from the perspective of downstream devices and the Spanning Tree Protocol (STP). To achieve this, the MCLAG primary switch synchronizes its Bridge MAC address and STP priority with the secondary switch. Consequently, both Core-1 and Core-2 share the exact same Bridge ID (Priority 20480 and MAC 02090f000701, as seen in the exhibits). Because they represent a single logical entity in the MSTP topology with the lowest shared bridge ID, both devices correctly report themselves as the root bridge.

A. FortiGate firewalls typically do not participate in switch-level STP. Furthermore, external participation would not cause two distinct switches to share an identical Bridge MAC address.

B. An incorrectly configured ISL would likely cause a split-brain scenario, but the exhibits explicitly show synchronized identical Bridge MACs, confirming proper logical operation, not an inconsistency.

D. The provided CLI exhibits clearly display active MSTP instance parameters, such as Forward Time and Max Age, directly contradicting the assertion that MCLAG automatically disables STP.

Fortinet, Inc. FortiSwitchOS Administration Guide. "Multi-chassis link aggregation groups (MCLAG)" -> "MCLAG and spanning tree protocol (STP)". The documentation specifies that "the MCLAG peer group acts as a single STP node. Both MCLAG peer switches share the same bridge ID."