CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Remember that STP prioritizes root path cost first, not bridge or port IDs. Misunderstanding the order of evaluation can lead to incorrect answers.
Answer choices
Why each option matters
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.
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
The local interface MAC address is not compared when selecting a root port; STP evaluates the BPDU content arriving on each port, prioritizing the root bridge ID, root path cost, and the upstream bridge/port IDs. The switch's own MAC address is used only in the bridge ID to identify itself, not as a tie-breaker for choosing which of its ports becomes the root port. Thus, a lower local MAC address provides no advantage.
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
The spanning-tree root port election first compares the cumulative root path cost advertised in each received BPDU. The port with the lowest root path cost to the root bridge is selected as the root port, because it offers the most efficient shortest path toward the root. Only when costs are equal does STP proceed to other tie-breakers such as the sender bridge ID and sender port ID.
- ✗
Highest duplex setting
Why it's wrong here
Duplex setting is never a component of the STP root port decision process. While a duplex mismatch on a point-to-point link can cause late collisions and BPDU loss, the IEEE 802.1D specification does not consider interface duplex when electing a root port. Therefore, the highest duplex value is irrelevant, and the correct decision is based solely on the BPDU cost and sender parameters.
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.
- ✗
Lowest configured VLAN number
Why it's wrong here
The configured VLAN number is irrelevant to root port election because STP operates identically within each VLAN or MST instance. Although a separate spanning-tree instance exists for each VLAN, the election criteria (root path cost, sender bridge ID, sender port ID) are the same regardless of the VLAN ID. The VLAN number only determines which forwarding database and instance is used, not which port becomes the root port.
When this WOULD be correct
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
The spanning-tree root port election first compares the cumulative root path cost advertised in each received BPDU. The port with the lowest root path cost to the root bridge is selected as the root port, because it offers the most efficient shortest path toward the root. Only when costs are equal does STP proceed to other tie-breakers such as the sender bridge ID and sender port ID.
✗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?”
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
Root Guard, Loop Guard, and BPDU Guard in Rapid PVST+
Key term
Root port
A root port is the port on a non-root switch in a Spanning Tree Protocol network that has the lowest cost path to the root bridge.
Key term
Path cost
Path cost is a numerical value assigned to each network path used by the Spanning Tree Protocol to determine the most efficient route and prevent loops in a switched network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.