The answer is the lowest root path cost. STP root port selection path cost is the primary factor because the switch must determine which uplink provides the most efficient route to the root bridge, and path cost directly reflects the cumulative bandwidth delay along that path. On the CCNA 200-301 v2 exam, this concept tests your understanding of the Spanning Tree Protocol decision sequence, often appearing in exhibits where two uplinks receive superior BPDUs—the switch compares root path costs first, and only if they tie does it examine sender bridge ID and then sender port ID. A common trap is assuming the sender bridge ID or port priority is checked first, but the cost always takes precedence. Remember the mnemonic: "Cost comes first, then the bridge, then the port."
CCNA Switching and Network Access Practice Question
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 selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.. 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.
Exhibit
SW2 uplink A root path cost = 4
SW2 uplink B root path cost = 19
Both links receive superior BPDUs
Exhibit: SW2 receives superior BPDUs on both uplinks. One uplink becomes the root port and the other becomes alternate. Which factor is considered first when SW2 chooses the root port?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Lowest root path cost
STP chooses the best path to the root bridge based first on the lowest root path cost. If the cost ties, it then checks the sender bridge ID and sender port ID as tie-breakers.
Key principle: STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Lowest local interface MAC address
Why it's wrong here
MAC address is not the first deciding factor for root port selection.
When this WOULD be correct
In a different question setup where the focus is on determining the primary interface for a device based on MAC addresses (e.g., a scenario involving MAC address-based load balancing), the lowest local interface MAC address could be the correct criterion for selection.
✓
Lowest root path cost
Why this is correct
That is the first major comparison.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
✗
Highest duplex setting
Why it's wrong here
Duplex is not a root port tie-breaker.
When this WOULD be correct
In a different question context where the focus is on interface capabilities, such as determining which port to use for data transmission based on performance metrics, the highest duplex setting could be considered. For example, a question might ask which port should be prioritized for traffic if all other factors are equal.
In a different scenario where the question asks about VLAN prioritization in a multi-VLAN environment, the lowest configured VLAN number could be relevant. For example, if the question focuses on how VLAN IDs impact traffic flow or STP behavior in a specific VLAN context, this option could be correct.
Option-by-option analysis
Why each answer is right or wrong
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.
✓Lowest root path costCorrect answer▾
Why this is correct
That is the first major comparison.
✗Lowest local interface MAC addressWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the selection of the root port is based on the lowest root path cost, not the local interface MAC address. The MAC address is not a factor in determining the root port in the Spanning Tree Protocol (STP) process.
★ When this WOULD be the correct answer
In a different question setup where the focus is on determining the primary interface for a device based on MAC addresses (e.g., a scenario involving MAC address-based load balancing), the lowest local interface MAC address could be the correct criterion for selection.
Why candidates choose this
Candidates may find this option tempting because they might confuse the MAC address with the criteria used in other networking protocols, leading them to believe it plays a role in STP root port selection.
✗Highest duplex settingWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the selection of the root port in Spanning Tree Protocol (STP) is based on the lowest root path cost, not the duplex settings of the interfaces. Duplex settings do not influence the port selection process in STP.
★ When this WOULD be the correct answer
In a different question context where the focus is on interface capabilities, such as determining which port to use for data transmission based on performance metrics, the highest duplex setting could be considered. For example, a question might ask which port should be prioritized for traffic if all other factors are equal.
Why candidates choose this
Candidates may find this option tempting because they might confuse physical layer characteristics, such as duplex settings, with logical layer decisions in network protocols, leading to an assumption that performance metrics influence port selection.
✗Lowest configured VLAN numberWrong answer — click to see why▾
Why this is wrong here
The lowest configured VLAN number is not a factor in determining the root port in Spanning Tree Protocol (STP). The selection process prioritizes path cost, not VLAN configuration.
★ When this WOULD be the correct answer
In a different scenario where the question asks about VLAN prioritization in a multi-VLAN environment, the lowest configured VLAN number could be relevant. For example, if the question focuses on how VLAN IDs impact traffic flow or STP behavior in a specific VLAN context, this option could be correct.
Why candidates choose this
Candidates may confuse VLAN configuration with STP operations, mistakenly believing that VLAN IDs influence port selection. This confusion can lead them to select this option, thinking it relates to network topology management.
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
Common exam trap: answer the scenario, not the keyword
Remember that STP prioritizes root path cost first, not bridge or port IDs. Misunderstanding the order of evaluation can lead to incorrect answers.
Detailed technical explanation
How to think about this question
Spanning Tree Protocol (STP) is designed to prevent Layer 2 loops in Ethernet networks by creating a loop-free logical topology. Each switch in the STP domain elects a root bridge, which is the logical center of the network. Non-root switches determine their best path to the root bridge by selecting a single root port. This root port is the port with the lowest cumulative cost to reach the root bridge, known as the root path cost. The root path cost is calculated by adding the cost values of each link along the path to the root bridge, where lower cost links are preferred.
When a switch receives superior BPDUs on multiple ports, it compares the root path cost advertised in those BPDUs to select the root port. The port with the lowest root path cost becomes the root port. If two or more ports have the same root path cost, STP uses tie-breakers in a specific order: first, the sender bridge ID (the MAC address and priority of the switch sending the BPDU) is compared, and if still tied, the sender port ID is compared. These tie-breakers ensure a deterministic and loop-free topology. Local interface MAC addresses, duplex settings, or VLAN numbers are not factors in root port selection.
A common exam trap is to confuse the root port selection criteria with local interface characteristics such as MAC address or duplex settings. Candidates might incorrectly assume the lowest MAC address or highest duplex port is chosen first, but STP strictly follows root path cost and bridge/port IDs. Practically, the alternate port is a blocked port that provides redundancy; it becomes the root port only if the current root port fails. Understanding this selection process is critical for troubleshooting STP issues and ensuring network stability in Cisco environments.
KKey Concepts to Remember
STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
The root path cost is the cumulative cost of all links from the switch to the root bridge, calculated using standard STP cost values.
If multiple ports receive superior BPDUs with the same root path cost, STP uses the sender bridge ID as the first tie-breaker.
If the sender bridge ID is also equal, STP then compares the sender port ID to break ties and select the root port.
STP does not consider local interface MAC addresses or duplex settings when selecting the root port.
VLAN numbers do not influence root port selection because STP operates per VLAN but chooses root ports based on path cost and bridge IDs.
An alternate port is a backup port that is blocked to prevent loops but can become the root port if the current root port fails.
STP ensures a loop-free topology by selecting one root port per switch and blocking redundant paths using alternate ports.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review sTP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge..
What is the correct answer to this question?
The correct answer is: Lowest root path cost — STP chooses the best path to the root bridge based first on the lowest root path cost. If the cost ties, it then checks the sender bridge ID and sender port ID as tie-breakers.
What should I do if I get this 200-301 question wrong?
Review sTP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge., then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
STP selects the root port on a non-root switch as the port with the lowest root path cost to the root bridge.
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