Quick Answer
The correct answer is that STP is designed to prevent Layer 2 switching loops, and the root bridge is elected based on the lowest bridge ID. STP prevents loops by dynamically blocking redundant paths, placing some switch ports into a non-forwarding role to ensure a single logical path exists between any two network segments. The root bridge election process uses the bridge ID, which combines a bridge priority value with the switch’s MAC address; the switch with the numerically lowest bridge ID wins the election, and all its ports automatically become designated ports for their respective segments. On the CCNA 200-301 v2 exam, this concept tests your understanding of loop prevention fundamentals and the deterministic election process, often appearing in multiple-choice questions that pair a correct statement about loop prevention with a correct statement about root bridge selection. A common trap is assuming the root bridge is chosen by highest bandwidth or lowest MAC address alone, but the bridge ID as a whole is the deciding factor. Remember the mnemonic "Lowest BID wins the ID" to recall that the lowest bridge ID determines the root.
This 200-301 practice question tests your understanding of switching and network access. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: sTP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which two STP facts are correct? Choose two.
The root bridge is elected using the lowest bridge ID
That is how the root bridge is selected.
STP is designed to prevent Layer 2 switching loops
Loop prevention is the whole reason STP exists.
A lower path cost makes a port less likely to become root port
Why wrong: Lower cost is exactly what a switch prefers toward the root.
All blocked ports are designated ports
Why wrong: Blocked ports are not designated forwarding ports.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The root bridge is elected using the lowest bridge ID
STP prevents Layer 2 loops by placing some ports into a non-forwarding role. The root bridge is chosen by the lowest bridge ID, and all ports on the root bridge itself are designated ports for their segments.
Key principle: STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The root bridge is elected using the lowest bridge ID
Why this is correct
That is how the root bridge is selected.
Related concept
STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.
STP is designed to prevent Layer 2 switching loops
Why this is correct
Loop prevention is the whole reason STP exists.
Related concept
STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.
A lower path cost makes a port less likely to become root port
Why it's wrong here
Lower cost is exactly what a switch prefers toward the root.
When this WOULD be correct
In a question about STP port roles where the statement is 'A higher path cost makes a port less likely to become root port', that would be correct because higher cost is less preferred.
All blocked ports are designated ports
Why it's wrong here
Blocked ports are not designated forwarding ports.
When this WOULD be correct
If the question asked 'Which ports are in blocking state in STP?' and an option said 'All non-designated ports are blocked', that would be correct, as blocked ports are a subset of non-designated ports.
Option-by-option analysis
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
Why this is correct
That is how the root bridge is selected.
Why this is wrong here
STP uses path cost to determine the root port, where a lower path cost is more desirable and makes a port more likely to become the root port, not less.
★ When this WOULD be the correct answer
In a question about STP port roles where the statement is 'A higher path cost makes a port less likely to become root port', that would be correct because higher cost is less preferred.
Why candidates choose this
Candidates may confuse the relationship between path cost and port role, thinking that lower cost is somehow less favorable or misremembering the election criteria.
Why this is wrong here
In STP, blocked ports are alternate or backup ports, not designated ports. Designated ports are in forwarding state on each segment, while blocked ports are non-designated.
★ When this WOULD be the correct answer
If the question asked 'Which ports are in blocking state in STP?' and an option said 'All non-designated ports are blocked', that would be correct, as blocked ports are a subset of non-designated ports.
Why candidates choose this
Candidates may confuse the roles of ports in STP, thinking that all ports that are not forwarding (including blocked) are designated, or they may misremember that designated ports are always forwarding.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Be careful not to confuse root ports with designated ports, and remember that STP uses path cost, not hop count.
Detailed technical explanation
Spanning Tree Protocol (STP) is a Layer 2 network protocol designed to prevent switching loops in Ethernet networks by selectively blocking redundant paths. STP elects a root bridge, which serves as a reference point for all path calculations. Each switch uses the root bridge ID, a combination of priority and MAC address, to determine the root bridge, with the lowest bridge ID winning the election. This root bridge then influences port roles and forwarding decisions across the network. STP assigns port roles based on path cost to the root bridge. Each non-root switch selects a single root port, which is the port with the lowest cumulative cost path back to the root bridge. Ports on the root bridge itself are always designated ports, forwarding traffic for their segments. Ports that could cause loops are placed into a blocking state and are not designated ports. This mechanism ensures a loop-free topology while maintaining redundancy. A common exam trap is misunderstanding the relationship between port cost and port roles. Lower path cost ports are more likely to become root ports, not less, which contradicts option C. Also, blocked ports are never designated ports, as option D incorrectly states. Understanding these distinctions is critical for correctly answering STP questions and for practical network design, where proper port roles ensure stability and redundancy without loops.
Key takeaway
STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Review sTP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Switching and Network Access — This question tests Switching and Network Access — STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address..
The correct answer is: The root bridge is elected using the lowest bridge ID — STP prevents Layer 2 loops by placing some ports into a non-forwarding role. The root bridge is chosen by the lowest bridge ID, and all ports on the root bridge itself are designated ports for their segments.
Review sTP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address., then practise related 200-301 questions on the same topic to reinforce the concept.
STP elects the root bridge by selecting the switch with the lowest bridge ID, which combines priority and MAC address.
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Last reviewed: Apr 13, 2026
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