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Difficulty: MediumSpanning Tree Protocol Operations

Four switches are deployed in a single LAN segment running IEEE 802.1D Spanning Tree Protocol (STP). The bridge configuration settings for each switch are as follows:

- Switch-1: Bridge Priority = 3276832768, MAC Address = `00:11:22:33:44:55`
- Switch-2: Bridge Priority = 40964096, MAC Address = `00:AA:BB:CC:DD:EE`
- Switch-3: Bridge Priority = 81928192, MAC Address = `00:11:22:11:22:33`
- Switch-4: Bridge Priority = 40964096, MAC Address = `00:AA:BB:CC:DD:AA`

When the switches finish initialization, which switch is elected as the STP Root Bridge, and for what reason?

  1. A
    Switch-1, because a higher numerical priority value grants superior election precedence in standard STP operations.
  2. B
    Switch-3, because its MAC address is numerically lower than all other switches regardless of priority configuration.
  3. C
    Switch-2, because it has a priority value of 40964096 and its MAC address is evaluated first in alphabetical sequence.
  4. Switch-4, because it ties for the lowest bridge priority of 40964096 and possesses the lower MAC address tie-breaker.Answer

Answer

Switch-4 is elected as the Root Bridge because it ties for the lowest priority value (40964096) and has a lower MAC address (`00:AA:BB:CC:DD:AA`) than Switch-2 (`00:AA:BB:CC:DD:EE`).
In IEEE 802.1D Spanning Tree Protocol, the Root Bridge election process evaluates the Bridge Identifier (BID), which combines a 2-byte Bridge Priority and a 6-byte MAC address. The switch with the lowest BID becomes the Root Bridge. Priority is evaluated first; both Switch-2 and Switch-4 have the lowest priority (40964096). To break the tie, STP compares MAC addresses. Comparing `00:AA:BB:CC:DD:AA` (Switch-4) and `00:AA:BB:CC:DD:EE` (Switch-2), Switch-4 has the smaller MAC address and is successfully elected as the Root Bridge.

Step-by-Step Solution

1
Evaluate Bridge Priority values for all candidate switches.
Switch-2 (40964096) and Switch-4 (40964096) share the lowest numerical bridge priority value.
STP Root Bridge election strictly prioritizes the lowest Bridge Priority (Bridge ID = Priority + MAC Address).
2
Compare MAC addresses as a tie-breaker between Switch-2 and Switch-4.
Switch-4 (`00:AA:BB:CC:DD:AA`) is numerically lower than Switch-2 (`00:AA:BB:CC:DD:EE`).
When bridge priorities are identical, the switch with the lowest MAC address breaks the tie to determine the Root Bridge.

Key Concept

STP Root Bridge Election Order (Priority then MAC Address)
Estimated Time:1m 15s
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Topics

Spanning Tree Protocol Operations

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