The SR-series platform uses a concept of M-VPLS to group different SAPs under a single STP instance. The VLAN range covering SAPs to be managed by a specified M-VPLS is declared under a specific M-VPLS SAP definition. MSTP mode-of-operation is only supported in an M-VPLS.
When running MSTP, by default, all VLANs are mapped to the CIST. At the VPLS level, VLANs can be assigned to specific MSTIs. When running RSTP, the operator must explicitly indicate, per SAP, which VLANs are managed by that SAP.