http://www.juniper.net/techpubs/en_US/junos15.1/topics/example/vstp_on_qfx_example.html
And one more thing:
 | Note: When configuring xSTP on EX4300 and EX4600 switches, you must add all the interfaces in the applied VLANs in configurations. For MSTP, configure all interfaces in all VLANs at the [edit protocols mstp interface] hierarchy level. |