The bridge priority is a customizable value that you can use to
influence which switch becomes the root bridge. The switch with the
lowest priority, which means lowest BID, becomes the root bridge (the
lower the priority value, the higher the priority). The default value
for the priority of all Cisco switches is 32768. The priority range is
between 1 and 65536; therefore, 1 is the highest priority.
Now have a look at this diagram :
The
early implementation of STP was designed for networks that did not use
VLANs. There was a single common spanning tree across all switches. When
VLANs started became common for network infrastructure segmentation,
STP was enhanced to include support for VLANs. As a result, the extended
system ID field contains the ID of the VLAN with which the BPDU is
associated.
When
the extended system ID is used, it changes the number of bits available
for the bridge priority value, so the increment for the bridge priority
value changes from 1 to 4096. Therefore, bridge priority values can
only be multiples of 4096.
Note that 2 raise to power 12 is 4096. Now if you occupy even a single bit ( out of the 4 bits) for the Bridge Priority,
It means 4096*2=8192 (multiple of 4096..)
The
extended system ID value is added to the bridge priority value in the
BID to identify the priority and VLAN of the BPDU frame.
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