The value of the priority component of the Bridge
ID for the corresponding Spanning Tree instance.
Specifically, the four most significant bits of the
most significant octet of the (8 octet long) Bridge
ID followed by 12 zero bits. Note that this object
may only be set to a 16-bit value in which the least
significant 12 bits are zero, i.e., permissible values
are 0-61440, in steps of 4096.