In the context of the topology provided and the concepts of LAG (Link Aggregation Group) and MCLAG (Multi-Chassis Link Aggregation), the spanning tree protocol's perspective can be summarized as follows:
Switch 1, Switch 2, and Switch 3 are seen as one MCLAG peer group (Option A): In this configuration, Switches 1 and 2 form a Multi-Chassis Link Aggregation Group (MCLAG) which effectively allows them to act as a single logical entity from the perspective of downstream switches (in this case, Switch 3). This grouping enhances fault tolerance and bandwidth by pooling the link resources of the two switches.
Switch 3 and Switch 4 uplinks are treated as single interfaces (Option B): This option suggests that the connections between Switch 3 and Switch 4 (presumably using LAG) are perceived by the spanning tree protocol as a single logical connection. This perception is due to the LAG configuration, which combines multiple network cables/ports into a single logical link to provide redundancy and increase bandwidth.
Reference:
The use of LAG and MCLAG is well-documented in networking literature and Fortinet's own documentation, as these technologies are commonly employed to enhance redundancy and bandwidth. Fortinet's implementation of these protocols is designed to maintain compatibility with standard networking protocols, including Spanning Tree Protocol (STP).