200-301 · Question #1544
Refer to the exhibit. Which switch in this configuration will be elected as the root bridge?
The correct answer is D. SW4: 0C:0A:05:22:05:97. In an STP domain, the switch with the lowest Bridge ID, determined by bridge priority and then MAC address, is elected as the root bridge.
Question
Refer to the exhibit. Which switch in this configuration will be elected as the root bridge?
Exhibits
Options
- ASW1: 0C:4A:82.:65:62:72
- BSW2: 0C:0A:A8:1A:3C:9D
- CSW3: 0C:0A:18:81:B3:19
- DSW4: 0C:0A:05:22:05:97
How the community answered
(49 responses)- A4% (2)
- B4% (2)
- C8% (4)
- D84% (41)
Why each option
In an STP domain, the switch with the lowest Bridge ID, determined by bridge priority and then MAC address, is elected as the root bridge.
SW1's MAC address (0C:4A:82:65:62:72) is numerically higher than SW4's MAC address, meaning it would not be elected root bridge with default priorities.
SW2's MAC address (0C:0A:A8:1A:3C:9D) is numerically higher than SW4's MAC address when comparing the third octet ('A8' vs '05'), meaning it would not be elected root.
SW3's MAC address (0C:0A:18:81:B3:19) is numerically higher than SW4's MAC address when comparing the third octet ('18' vs '05'), meaning it would not be elected root.
In Spanning Tree Protocol, the root bridge is elected by comparing the Bridge ID of each switch. Assuming default bridge priorities, the switch with the numerically lowest MAC address is chosen as the root bridge. Comparing the provided MAC addresses, SW4 (0C:0A:05:22:05:97) has the lowest MAC address, making it the root bridge.
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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