Spanning Tree - Bridge Priority

Bridge ID consist of 8 Bytes and it is divided into two parts consisting of Bridge priority and Switch Mac Address.

To accommodate the additional VLAN information, Extended System ID field was introduced, borrowing 12 bits from the original Bridge Priority

Bridge Priority Field can only be set in increments of 4096.

This means that the possible values are : 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768 etc.

By default, Cisco’s Per-VLAN Spanning-Tree Plus (PVST+) adds this System ID Extension (sys-id-ext) to the Bridge Priority.

The two values (Bridge Priority + System ID Extension) together make up the Bridge ID which is used to elect the Root Bridge.