A.Session layer

B.Network layer

C.Infrastructure layer

D.Application layer

A.Calculating CST only needs the value of RSTBPDU field

B.If the MST region is regarded as a logically running RSTP switch, the CIST external path cost is equivalent to the root path cost of the RSTP switch

C.MSTP BPDU consists of RSTBPDU field and MST specific field

D.If the MST region is regarded as a switch that logically runs RSTP, the CIST region root ID is equivalent to the designated bridge ID of the RSTP switch, usually the ID is the bridge ID with the smallest value in the MST region