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Routing & Switching · Foundational

Bridge ID (BID)

An 8-byte STP identifier made of a 2-byte priority (with extended system ID) and the switch MAC address.

Definition

The priority field defaults to 32768 and must be a multiple of 4096 because the lower 12 bits carry the extended system ID (the VLAN number). The lowest bridge ID wins root election; with equal priorities, the lowest MAC wins.

Example

Switch priority 32778 for VLAN 10 = 32768 + 10.

Exam tip

Priority must be in increments of 4096 because of the 12-bit extended system ID.

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