Trunking FortiSwitch to Cisco Switch

Hello,

As per the query you have addressed you can use the link mentioned below 

Inter-operability with rapid Per-VLAN Spanning Tree plus (PVST+) can be enabled per port on managed FortiSwitch devices. It is disabled by default.

https://docs.fortinet.com/document/fortigate/6.4.0/new-features/499186/inter-operability-with-per-instance-rstp-802-1w-6-4-2

https://docs.fortinet.com/document/fortiswitch/7.0.1/administration-guide/364618/support-for-interoperation-with-rapid-per-vlan-rstp-rapid-pvst-or-rpvst

Be aware of the limit of  the ussage of 15 Vlans + Native Vlan (untagged) if you are configure interoperabity to Pvst+. 

e.g. 

   edit "port3"                 set rpvst-port enable

We are working at the same issue with MST. It is not possible to configure the default (Region name = NULL and Revsion number=0) at Cisco side. At fortinet side, it is  not possible to set vlan-range at instance 0 !!!! Try to define a new region name with a new revison number. Think about to create 2 Instances analog to Fortigate Defaults (Instance 0 = Vlan 1-4093 and Instance 15 = Vlan 4093). Hope thats help you. Be aware that regio name and revision number have to be the same, between Cisco and Fortigate. 

Also to discuss: MST Configuration Digest - is the algorithm the same between Cisco and Fortigate ? How knows.

config switch stp instance
edit "0"

!  set vlan-range ! not possible

config switch stp instance
edit "15"

set vlan-range 4094

set vlan-range
<vlan_map> single vlans or ranges of vlans separated by commas(no whitespace)
e.g. 1,3-4,6,7,9-100
The vlan_map configuration entered must be less than 4096 characters.