200-301 · Question #1543
Refer to the exhibit. Which switch in this configuration will be elected as the root bridge? SW1: 0C:E4:82:33:62:23 SW2: 0C:0E:16:11:05:97 SW3: 0C:E0:16:1A:3C:9D SW4: 0C:00:18:A1:B3:19
The correct answer is B. SW2. The root bridge in an STP domain is elected based on the lowest Bridge ID, which combines bridge priority and MAC address.
Question
Exhibits
Options
- ASW1
- BSW2
- CSW3
- DSW4
How the community answered
(52 responses)- A6% (3)
- B73% (38)
- C17% (9)
- D4% (2)
Why each option
The root bridge in an STP domain is elected based on the lowest Bridge ID, which combines bridge priority and MAC address.
SW1's MAC address (0C:E4:82:33:62:23) is numerically higher than SW2's, so it would not be elected root with default priorities.
In Spanning Tree Protocol, the root bridge is elected based on the lowest Bridge ID. While not explicitly stated with default priorities, for SW2 to be elected as the root bridge over SW4 which has a numerically lower MAC address, SW2 must possess the lowest Bridge ID, typically achieved by configuring a lower Bridge Priority value than the other switches in the network.
SW3's MAC address (0C:E0:16:1A:3C:9D) is numerically higher than SW2's, so it would not be elected root with default priorities.
SW4's MAC address (0C:00:18:A1:B3:19) is numerically lower than SW2's, meaning it would be the root if all switches had default priorities, but given B is the correct answer, SW4's Bridge ID must be higher (e.g., due to a higher priority).
Concept tested: Spanning Tree Protocol (STP) Root Bridge Election
Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html
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