A full mesh topology, where every FortiSwitch connects to every other FortiSwitch, inherently creates multiple redundant Layer 2 paths. Without a loop-prevention mechanism, this design would lead to broadcast storms and MAC address table instability, rendering the network unusable. The Spanning Tree Protocol (STP) is the industry-standard protocol designed specifically to address this issue. STP identifies redundant links and logically blocks them to create a single, loop-free path between any two network devices. It maintains the physical redundancy, allowing a blocked path to become active if the primary path fails, thus ensuring both connectivity and a loop-free environment.

A. Multichassis link aggregation group (MCLAG) is a feature for device-level redundancy, typically between two core switches, not a general loop-prevention protocol for a full mesh topology.

C. Full mesh high availability (HA) describes the physical network design and its goal, not the specific protocol or feature configured to prevent loops within that design.

D. A link aggregation group (LAG) bundles multiple physical links into a single logical link for bandwidth and redundancy but does not prevent loops formed between separate switches.

1. Fortinet FortiSwitchOS 7.6.0 Administration Guide, Page 101, "Spanning Tree Protocol (STP)" section: "The Spanning Tree Protocol (STP) is a link-management protocol that provides path redundancy while preventing undesirable loops in a network. For an Ethernet network to function properly, only one active path can exist between any two stations. STP prevents loops by blocking redundant links." This directly confirms STP's role in preventing loops in redundant topologies.

2. Fortinet FortiSwitchOS 7.6.0 Administration Guide, Page 113, "Multichassis Link Aggregation (MCLAG)" section: This section describes MCLAG as a feature to provide redundancy by allowing a client to connect to two different FortiSwitch units. While it has loop-prevention mechanisms for its specific topology, it is not the fundamental protocol for a generic full mesh. The guide states, "STP is still required to provide a loop-free topology with other switches in the network." This highlights that STP is the underlying requirement.