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Difficulty: MediumSpanning Tree Protocol Operations

Four network switches (SW1, SW2, SW3, SW4) are connected in a local area network running standard IEEE 802.1D Spanning Tree Protocol (STP). The bridge priorities and base MAC addresses for the switches are configured as follows:

- SW1: Bridge Priority = 3276832768, MAC = 0200.1111.11110200.1111.1111
- SW2: Bridge Priority = 3276832768, MAC = 0200.2222.22220200.2222.2222
- SW3: Bridge Priority = 1638416384, MAC = 0200.3333.33330200.3333.3333
- SW4: Bridge Priority = 3276832768, MAC = 0200.0000.11110200.0000.1111

Which switch will be elected as the STP root bridge in this network topology?

  1. SW3, because it possesses the lowest numerical bridge priority value.Answer
  2. B
    SW4, because it possesses the lowest numerical MAC address among all switches.
  3. C
    SW2, because default priority ties force the election to select the highest MAC address.
  4. D
    SW1, because it retains the default bridge priority and lowest MAC among default-configured switches.

Answer

SW3 is elected as the root bridge because it has the lowest Bridge Priority (16384), making its overall Bridge ID the lowest in the network.
In 802.1D Spanning Tree Protocol, the switch with the lowest Bridge Identifier (BID) becomes the root bridge. The BID is composed of the Bridge Priority and the MAC address. Bridge Priority is compared first; the lower numerical value wins. Since SW3 has a Bridge Priority of 16384 while SW1, SW2, and SW4 all have the default priority of 32768, SW3 wins the root bridge election automatically without needing a MAC address tie-breaker.

Step-by-Step Solution

1
Understand the Spanning Tree Protocol (STP) Bridge ID structure.
Bridge ID (BID) is calculated as: BID=Bridge Priority+MAC Address\text{BID} = \text{Bridge Priority} + \text{MAC Address}.
STP elects the switch with the lowest numerical BID as the root bridge.
2
Compare the Bridge Priority values of all participating switches.
SW1 = 32768, SW2 = 32768, SW3 = 16384, SW4 = 32768.
Bridge Priority is evaluated before MAC addresses.
3
Identify the switch with the lowest priority.
SW3 has a priority of 16384, which is lower than 32768.
Since SW3 has the lowest priority, MAC address tie-breaking is not needed for SW3.

Key Concept

STP Root Bridge Election Process
Estimated Time:1m 0s
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Topics

Spanning Tree Protocol Operations

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