200-301 · Question #827
Which two statements about the spanning-tree bridge ID are true? (Choose two.)
The correct answer is A. It is composed of a 4-bit bridge priority and a 12-bit system ID extension. E. The bridge priority must be incremented in blocks of 4096.. The Spanning Tree Protocol (STP) Bridge ID comprises a 4-bit bridge priority and a 12-bit system ID extension, and its priority is configured in increments of 4096.
Question
Which two statements about the spanning-tree bridge ID are true? (Choose two.)
Options
- AIt is composed of a 4-bit bridge priority and a 12-bit system ID extension.
- BThe bridge ID is transmitted in the IP header to elect the root bridge.
- CThesystem ID extension is a value between 1 and 4095.
- DIt is composed of an 8-bit bridge priority and a 16-bit system ID extension.
- EThe bridge priority must be incremented in blocks of 4096.
How the community answered
(23 responses)- A87% (20)
- C9% (2)
- D4% (1)
Why each option
The Spanning Tree Protocol (STP) Bridge ID comprises a 4-bit bridge priority and a 12-bit system ID extension, and its priority is configured in increments of 4096.
The STP Bridge ID is composed of a 4-bit bridge priority and a 12-bit system ID extension (typically for VLAN ID), which together form a 16-bit value that is concatenated with the switch's MAC address.
The bridge ID is transmitted in Spanning Tree Protocol (STP) Bridge Protocol Data Units (BPDUs) at Layer 2, not within the IP header at Layer 3.
While the system ID extension often corresponds to VLAN IDs (1-4094), the 12-bit field itself technically covers values from 0 to 4095; however, stating '1 to 4095' is less accurate than the specific increment of bridge priority.
The bridge priority is 4 bits and the system ID extension is 12 bits, not 8 bits and 16 bits, respectively, which would total 24 bits instead of the correct 16 bits before the MAC address.
The bridge priority component of the Bridge ID must be configured in increments of 4096 (e.g., 0, 4096, 8192) to allow the 12-bit system ID extension to represent VLAN IDs in PVST+ and MSTP without overlapping priority ranges.
Concept tested: Spanning Tree Bridge ID Structure
Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/layer2/configuration_guide/b_layer2_3se_3850_cg/b_layer2_3se_3850_cg_chapter_010.html#concept_354B4B94C660467794DE70D90C84A3B7
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