200-301 · Question #1350
Refer to the exhibit. Which switch becomes the root bridge?
The correct answer is A. SW 3. The question asks to identify the root bridge in a Spanning Tree Protocol (STP) network, which is determined by the switch with the lowest Bridge ID.
Question
Options
- ASW 3
- BSW 2
- CSW 4
- DSW 1
How the community answered
(53 responses)- A72% (38)
- B4% (2)
- C17% (9)
- D8% (4)
Why each option
The question asks to identify the root bridge in a Spanning Tree Protocol (STP) network, which is determined by the switch with the lowest Bridge ID.
In a Spanning Tree Protocol (STP) topology, the switch with the lowest Bridge ID (BID) is elected as the root bridge. The BID is a combination of the bridge priority (default 32768) and the switch's MAC address; therefore, SW 3 must have the lowest BID among all switches, making it the root bridge.
SW 2 would only be the root bridge if it had a lower Bridge ID (priority + MAC address) than SW 3, SW 1, and SW 4.
SW 4 would only be the root bridge if it had a lower Bridge ID (priority + MAC address) than SW 3, SW 1, and SW 2.
SW 1 would only be the root bridge if it had a lower Bridge ID (priority + MAC address) than SW 3, SW 2, and SW 4.
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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