It came up very briefly at work today why the STP bridge priority must be a multiple of 4096 so I thought I would post my response on here.

There was only 2 fields in the original STP Bridge ID this was a 2 byte priority which allowed any value for the priority to be set from 0 to 65,535, followed by 6 bytes for the MAC address for those tie breaker situations.

But when multiple spanning trees instances started to appear on networks due to technologies such as PVST+ and MST this caused the switch to have a single BID for all the VLANs as it could not differentiate between the VLANs. So switch vendors like Cisco used a unique MAC address for each VLAN, but this caused a wastage of MAC addresses as each switch could have to reserve up to 4094 addresses if non standard VLANs were used (Im sure there must have been a limit to the amount each switch could reserve, but this was before my time so I don’t have any practical information on this).

Therefore to prevent the overuse of the MAC addresses they turned the 2 bytes which was used in the priority field of the Bridge ID into a 4 bit priority and used the remaining 12 bits for the vlan, the extra information which is used to carry the VLAN number is called the Extended System ID, and this process is sometimes called MAC address reduction as it reduces the number of reserved MAC addresses needed. The 12 bits of extra VLAN information allows support for 4096 VLANs, so there is full support for extended range VLANs. Therefore because of the use of the Extended System ID in the Bridge ID, there is only the first 4 bits of the original 2 byte number to be used for the bridge priority so it only allows multiplies of 4096.