fortiswitch spanning tree mode MSTP

In a Multiple Spanning Tree Protocol (MSTP) configuration with two main switches in MC-LAG, it is recommended to set different priorities for redundancy. For example, you can set FortiSwitch A with a priority of 4096 and FortiSwitch B with a priority of 8192 to establish a hierarchy. When managed by a FortiGate, the FortiGate does not need to be the root bridge, but you can set the spanning tree priority on the FortiGate itself to influence the spanning tree topology. Best practice is to configure the priorities strategically to ensure a stable and efficient spanning tree network, considering factors like redundancy, network performance, and fault tolerance.

In MCLAG-ICL topology, both the core FSWs will act as root Bridge. Both FSWs will have MCLAG mac address that will act as root Bridge mac address.

Please refer below links:-

https://community.fortinet.com/t5/FortiSwitch/Technical-Tip-Spanning-tree-in-LAG-and-MCLAG/ta-p/347877

https://docs.fortinet.com/document/fortiswitch/7.6.0/fortiswitchos-administration-guide/860027/mclag

From the STP treeʼs point of view, the MCLAG switches should not present themselves differently as a single MCLAG dual-homed virtual switch (accessed through an MCLAG trunk) and as a pair of STP running switches (accessed through asymmetric individual ports, typically in a ring topology). For example, the spanning tree with its root bridge outside of the MCLAG switches cannot connect to it through a dual-homed trunk on one side, while connecting to the MCLAG switches with asymmetric ports at the same time. Such configurations present a mixed view of the MCLAG switches to the STP instance and are not supported.