You can set a Spanning Tree port priority. Port priority is used to break a tie when choosing the root port for a bridge, in a case where the choice is between ports connected to the same bridge. The port with the lowest value will be elected.
Use this command to set a port priority:
set spantree portpri port-string priority [sid sid]
Valid priority values are 0–240 (in increments of 16) with 0 indicating high priority.
Valid sid values are 0–4094. If not specified, SID 0 will be assumed.