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Difficulty: MediumSpanning Tree Protocol Operations

An administrator interconnects three switches—Switch- Alpha, Switch-Beta, and Switch-Gamma—in a redundant triangular loop topology using standard IEEE 802.1D Spanning Tree Protocol (STP). All switches use their factory default configuration settings except for bridge priorities, which are assigned as follows:

• Switch-Alpha: Bridge Priority = 32768, MAC Address = 0011.2233.4455
• Switch-Beta: Bridge Priority = 28672, MAC Address = 0055.4433.2211
• Switch-Gamma: Bridge Priority = 40960, MAC Address = 0000.1111.2222

Which switch will be elected as the STP Root Bridge for the topology?

  1. Switch-Beta, because it has the lowest numerical Bridge Priority value.Answer
  2. B
    Switch-Gamma, because it has the highest numerical Bridge Priority value.
  3. C
    Switch-Gamma, because it possesses the lowest numerical MAC address in the network topology.
  4. D
    Switch-Alpha, because it uses the default Bridge Priority value of 32768.

Answer

Switch-Beta will be elected as the Root Bridge because it possesses the lowest overall Bridge ID, determined primarily by having the lowest numerical Bridge Priority value (28672).
In 802.1D Spanning Tree Protocol, the Root Bridge election compares the Bridge ID (BID) of each switch in the network. The BID is composed of Bridge Priority and MAC Address. The switch with the lowest overall numerical BID becomes the Root Bridge. Priority is evaluated first; because Switch-Beta is configured with a priority of 28672—which is numerically lower than 32768 (Switch-Alpha) and 40960 (Switch-Gamma)—it wins the election immediately.

Step-by-Step Solution

1
Examine the components of the Spanning Tree Protocol Bridge ID (BID).
The BID consists of a 2-byte Bridge Priority concatenated with a 6-byte MAC Address (Bridge ID = Priority + MAC Address).
STP uses the total BID value to elect the Root Bridge, where the lowest numerical value wins.
2
Compare the configured Bridge Priority values across all switches.
Switch-Alpha has a priority of 32768, Switch-Beta has a priority of 28672, and Switch-Gamma has a priority of 40960.
Bridge Priority is evaluated first before checking MAC addresses.
3
Identify the switch with the lowest numerical Bridge Priority.
Switch-Beta has the lowest priority (28672 < 32768 < 40960) and is elected Root Bridge without needing a MAC address tiebreaker.
Lower numerical values represent higher priority in standard 802.1D STP root bridge elections.

Key Concept

STP Root Bridge Election Mechanics
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Topics

Spanning Tree Protocol Operations

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