The topology is show as below.
The swithes are communicating with each other by layer-2 network, with the ports deny VLAN 1. All switches have configured VLAN 10, 20 and 30, and enabled the MSTP feature. Configure DIS1 as the main root of instance 1 (VLAN 10, 20) and the backup root of instance 2 (VLAN 30), and configure DIS2 as the main root of instance 2 and the backup root of instance 1. During ping test within the same network segment, the ping test sometimes passed and sometimes failed.