Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

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Spanning Tree Protocol Configuration Guide, Cisco Catalyst IE9300 Rugged Series Switches

Bridge ID and switch priority

Updated: March 2, 2026

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This topic explains the concept of Bridge ID and Switch Priority in IEEE 802.1D, describing how each switch and VLAN is uniquely identified and how the bridge ID is constructed.

The bridge ID is a unique identifier assigned to each switch and VLAN, as required by the IEEE 802.1D standard. The bridge ID controls the selection of the root switch.

  • Each switch must have a unique bridge ID for every configured VLAN.

  • The bridge ID is eight bytes long. The first two bytes represent the switch priority, and the remaining six bytes are derived from the switch MAC address.

  • With PVST+ and rapid PVST+, each VLAN is treated as a separate logical bridge, requiring a distinct bridge ID per VLAN.

Bridge ID components with extended system ID

The two bytes previously used for the switch priority are reallocated into a four-bit priority value and a twelve-bit extended system ID value equal to the VLAN ID.

Table 1. Bits and their value
Bit 16 Bit 15 Bit 14 Bit 13 Bit 12 Bit 11 Bit 10 Bit 9 Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1

The bridge ID includes these components:

  • Switch priority (2 bytes): 4-bit priority value

  • Extended system ID (2 bytes): 12-bit value equal to the VLAN ID

  • MAC address (6 bytes): Derived from the switch MAC address

The extended system ID, switch priority, and Spanning Tree MAC address together make each bridge ID unique for each VLAN.

Support for the extended system ID affects how you configure the root switch, secondary root switch, and VLAN switch priority. If you set a higher priority value, your switch is less likely to become the root switch. If you set a lower value, your switch is more likely to become the root switch.