Which of the following description of the principle of automatic protection switching of the standard SSM protocol are correct? (Multiple choice)
A . The network element broadcasts the clock quality information of the synchronization source to the downstream site through the S1 byte, and notifies the upstream station that the clock quality from the local network element is “not available for synchronization” by the S1 byte in the direction of the synchronization source.
B . According to the clock source priority table, the network element first selects the clock with the highest quality level from the available clock sources as the homologous source.
C . If there are multiple clock sources with the highest quality level, the network element selects the clock with the highest clock priority as the synchronization source.
D . The network element broadcasts the clock quality information and the clock source ID of the synchronization source through the downstream station between the S1 bytes, and notifies the upstream station that the clock quality from the local network element is "not available for synchronization" by the S1 byte in the direction of the synchronization source.
Answer: ABC