Automatically Inheriting Ports--MSTP Only
In an MSTP environment, whether you manually or automatically bind a port to an MSTI in an MSTP region, the switch automatically binds that port to the CIST.
The CIST handles BPDU processing for itself and all of the MSTIs; therefore, the CIST must inherit ports from the MSTIs in order to transmit and receive BPDUs. You can only delete ports from the CIST if it is no longer a member of an MSTI.
For more information about MSTP, see Multiple Spanning Tree Protocol.