200-301 · Question #1438
Refer to the exhibit. Which switch becomes the root bridge?
The correct answer is D. SW4. The switch with the lowest Bridge ID becomes the root bridge in a Spanning Tree Protocol (STP) topology.
Question
Refer to the exhibit. Which switch becomes the root bridge?
Options
- ASW3
- BSW2
- CSW1
- DSW4
How the community answered
(39 responses)- A8% (3)
- B3% (1)
- C10% (4)
- D79% (31)
Why each option
The switch with the lowest Bridge ID becomes the root bridge in a Spanning Tree Protocol (STP) topology.
SW3 would only become the root bridge if its Bridge ID were lower than all other switches, which is not the case if SW4 is the correct answer.
SW2 would only become the root bridge if its Bridge ID were lower than all other switches, which is not the case if SW4 is the correct answer.
SW1 would only become the root bridge if its Bridge ID were lower than all other switches, which is not the case if SW4 is the correct answer.
In Spanning Tree Protocol (STP), the root bridge is elected based on the lowest Bridge ID. The Bridge ID is composed of a configurable Bridge Priority value (default 32768) and the switch's MAC address. Therefore, SW4 must have either the lowest configured Bridge Priority or, in case of a tie in priority, the numerically lowest MAC address among all switches, making it the root bridge.
Concept tested: STP root bridge election
Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html#topic1
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