200-301 · Question #1451
Refer to the exhibit. Which switch becomes the root bridge?
The correct answer is D. SW 4. In Spanning Tree Protocol (STP), the root bridge is elected based on the lowest Bridge ID, which combines a configurable bridge priority with the switch's MAC address.
Question
Options
- ASW 1
- BSW2
- CSW 3
- DSW 4
How the community answered
(22 responses)- A5% (1)
- B14% (3)
- C5% (1)
- D77% (17)
Why each option
In Spanning Tree Protocol (STP), the root bridge is elected based on the lowest Bridge ID, which combines a configurable bridge priority with the switch's MAC address.
SW1 would not be the root bridge if SW4 has a lower Bridge ID, implying SW1's Bridge ID is higher.
SW2 would not be the root bridge if SW4 has a lower Bridge ID, implying SW2's Bridge ID is higher.
SW3 would not be the root bridge if SW4 has a lower Bridge ID, implying SW3's Bridge ID is higher.
The Spanning Tree Protocol (STP) elects a root bridge by comparing the Bridge IDs of all switches in the network. A Bridge ID consists of a bridge priority (default 32768) and the switch's MAC address; the switch with the numerically lowest Bridge ID becomes the root. Given the correct answer, Switch 4 (SW4) must have been configured with the lowest bridge priority or possessed the lowest MAC address among all switches in the exhibit, 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
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