What are the principles of synchronous Ethernet planning? (Multiple choice)

What are the principles of synchronous Ethernet planning? (Multiple choice)
A . When there is BITS or other high-precision external clock devices, it is recommended that network elements use tracking external clock sources
B. When there is no BITS or other high-precision external clock devices, it is recommended that network elements track the line clock source
C. The internal clock source is recommended as the lowest level clock source
D. When stopping the SSM protocol, do not configure the clock in a ring in the network. When starting the SSM protocol, the SSM protocol information of all network elements in the local network must be set to be consistent

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Answer: ABCD