Ref: Scaling Networks v6 Companion Guide
"STP
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Extended System ID
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Bridge Priority
The bridge priority is a customizable value that can be used to influence which switch becomes the root bridge. The switch with the lowest priority, which implies the lowest BID, becomes the root bridge because a lower priority value takes precedence.
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The default priority value for all Cisco switches is the decimal value 32768. The range is 0 to 61440, in increments of 4096. Therefore, valid priority values are 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. A bridge priority of 0 takes precedence over all other bridge priorities. All other values are rejected.