SAP priority allows a configurable tie-breaking parameter to be associated with a SAP. When configuration BPDUs are being received, the configured SAP priority is used in some circumstances to determine whether a SAP is designated or blocked. These are the values used for CIST when running MSTP for the 7450 ESS or 7750 SR.
In traditional STP implementations (802.1D-1998), this field is called the port priority and has a value of 0 to 255. This field is coupled with the port number (0 to 255 also) to create a 16-bit value. In the latest STP standard (802.1D-2004), only the upper 4 bits of the port priority field are used to encode the SAP priority. The remaining 4 bits are used to extend the port ID field into a 12-bit virtual port number field. The virtual port number uniquely references a SAP within the STP instance. See SAP virtual port number for more information about the virtual port number.
STP computes the actual SAP priority by taking the configured priority value and masking out the lower four bits. The result is the value that is stored in the SAP priority parameter. For example, if a value of 0 was entered, masking out the lower 4 bits would result in a parameter value of 0. If a value of 255 was entered, the result would be 240.
The default value for SAP priority is 128. This parameter can be modified within a range of 0 to 255; 0 being the highest priority. Masking causes the values actually stored and displayed to be 0 to 240, in increments of 16.
CLI syntax:
config>service>vpls>sap>stp#
priority stp-priority