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Difficulty: Very hardSwitching Concepts and VLANs

An enterprise network deployment uses Per-VLAN Spanning Tree Plus (PVST+) across three interconnected Layer 2 switches (SW1, SW2, and SW3). The switches are configured with the following parameters:

- SW1: Base bridge priority set to 3276832768, MAC address `00:11:22:33:44:55`
- SW2: Base bridge priority set to 2867228672, MAC address `00:AA:BB:CC:DD:EE`
- SW3: Base bridge priority set to 2867228672, MAC address `00:AA:BB:CC:DD:11`

No explicit per-VLAN priority override commands have been configured for VLAN 100. Which switch is elected as the Spanning Tree Protocol (STP) Root Bridge for VLAN 100, and what is its calculated priority field within its Bridge Identifier (BID)?

  1. SW3 is elected as the Root Bridge with a total BID priority of 28772.Answer
  2. B
    SW3 is elected as the Root Bridge with a total BID priority of 28672.
  3. C
    SW2 is elected as the Root Bridge with a total BID priority of 28772.
  4. D
    SW1 is elected as the Root Bridge with a total BID priority of 32868.

Answer

SW3 is elected as the Root Bridge for VLAN 100 with a total BID priority of 28772.
The Spanning Tree Protocol (STP) Root Bridge election evaluates the 8-byte Bridge Identifier (BID), which consists of a 2-byte Priority field and a 6-byte MAC address. In PVST+, the 2-byte Priority field is divided into a 4-bit Priority multiplier (in increments of 4096) and a 12-bit System ID Extension representing the VLAN ID. For VLAN 100, the effective priorities are: SW1 = 32768+100=3286832768 + 100 = 32868, SW2 = 28672+100=2877228672 + 100 = 28772, and SW3 = 28672+100=2877228672 + 100 = 28772. SW2 and SW3 tie with the lowest priority value of 28772. To break the tie, the MAC addresses are compared. Comparing `00:AA:BB:CC:DD:EE` (SW2) and `00:AA:BB:CC:DD:11` (SW3), SW3 has the lower MAC address because hexadecimal `11` is smaller than `EE`. Thus, SW3 becomes the Root Bridge for VLAN 100 with a total BID priority of 28772.

Step-by-Step Solution

1
Calculate the effective Bridge Priority for each switch for VLAN 100.
SW1 priority = 32768+100=3286832768 + 100 = 32868; SW2 priority = 28672+100=2877228672 + 100 = 28772; SW3 priority = 28672+100=2877228672 + 100 = 28772.
PVST+ incorporates the 12-bit System ID Extension (VLAN ID) into the 16-bit Priority field of the Bridge ID (BID).
2
Compare the calculated Bridge Priorities to find the lowest numerical value.
SW2 and SW3 tie with the lowest priority of 28772 (SW1 is eliminated with 32868).
STP selects the switch with the lowest Bridge Priority as the Root Bridge.
3
Break the tie between SW2 and SW3 by comparing their 48-bit MAC addresses.
SW3 (`00:AA:BB:CC:DD:11`) is lower than SW2 (`00:AA:BB:CC:DD:EE`).
When priority values are identical, STP uses the numerically lowest MAC address as the tiebreaker. Comparing byte-by-byte in hexadecimal: `11` (171017_{10}) is less than `EE` (23810238_{10}).

Key Concept

PVST+ System ID Extension and Root Bridge Election Priority/MAC Tiebreaker
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Topics

Switching Concepts and VLANs

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