CCNA Switching and Network Access Practice Question
When spanning tree elects a root bridge, which value is considered first?
⚠ Common exam trap
Remember that the bridge priority is evaluated before the MAC address in the root bridge election process.
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 bridge ID, which begins with priority
The root bridge is the switch with the lowest bridge ID. The bridge ID is made up of priority and MAC address, so priority is considered first, then MAC address if priorities tie.
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 MAC address only
Why it's wrong here
The MAC address is only the second and least-significant component of the bridge ID, used as a tiebreaker when two switches advertise the same priority. A switch with a low MAC address but a high priority (a numerically larger priority value) will lose to a switch with a lower priority and a higher MAC address. Therefore, considering the MAC address alone ignores the dominant priority field and misrepresents how the STP root bridge election actually works.
When this WOULD be correct
In a different question that asks specifically about the criteria for selecting a root bridge when multiple bridges have the same priority, the lowest MAC address would be the deciding factor. For example, if the question specifies that all bridges have the same priority, then the lowest MAC address would be the correct answer.
- ✗
Lowest bridge priority only
Why it's wrong here
A switch's bridge priority is just the first field in the bridge ID, not the entire election value. If two switches have the same priority, the STP election must then compare their MAC addresses to break the tie, because the BID is considered as a whole. Selecting a root based solely on the lowest priority would produce an indeterminate result whenever priorities are equal, so priority alone is insufficient for the root bridge decision.
When this WOULD be correct
In a different question that asks specifically for the criteria used to determine the root bridge when only considering priority values, option B could be correct if the question states that all bridges have the same MAC address, thus making the lowest bridge priority the deciding factor.
- ✓
Lowest bridge ID, which begins with priority
Why this is correct
In STP, the root bridge is elected by comparing the 8-byte bridge ID (BID), which is formed by a 2-byte priority value followed by the 6-byte MAC address. The lowest BID wins, and because priority occupies the most significant bytes, a switch with a lower priority always beats a switch with a higher priority. Only when priorities are identical does the MAC address become the tiebreaker within the same BID comparison. Thus the actual election value is the full bridge ID, beginning with priority.
- ✗
Highest interface bandwidth
Why it's wrong here
Interface bandwidth is used to calculate STP path cost, which determines the best port on each non-root switch to reach the root bridge, but it has no role in choosing the root bridge itself. The root bridge is chosen by the lowest bridge ID, not by link speed. A high-bandwidth port does not make a switch more eligible to become the root, and any association between bandwidth and root election is a misunderstanding of STP's BID-based decision process.
When this WOULD be correct
In a different question that asks about factors influencing network performance or path selection in a Layer 2 network, the highest interface bandwidth could be relevant. For example, a question might ask which attribute affects the best path selection for data traffic in a network topology.
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 bridge ID, which begins with priorityCorrect answer▾
Why this is correct
In STP, the root bridge is elected by comparing the 8-byte bridge ID (BID), which is formed by a 2-byte priority value followed by the 6-byte MAC address. The lowest BID wins, and because priority occupies the most significant bytes, a switch with a lower priority always beats a switch with a higher priority. Only when priorities are identical does the MAC address become the tiebreaker within the same BID comparison. Thus the actual election value is the full bridge ID, beginning with priority.
✗Lowest MAC address onlyWrong answer — click to see why▾
Why this is wrong here
The MAC address is only used as a tiebreaker when bridge priorities are equal. It is not the first value considered in root bridge election.
★ When this WOULD be the correct answer
In a different question that asks specifically about the criteria for selecting a root bridge when multiple bridges have the same priority, the lowest MAC address would be the deciding factor. For example, if the question specifies that all bridges have the same priority, then the lowest MAC address would be the correct answer.
Why candidates choose this
Students may confuse the role of MAC address in bridge ID with it being the primary criterion, especially since MAC addresses are unique and often used in other networking decisions.
✗Lowest bridge priority onlyWrong answer — click to see why▾
Why this is wrong here
The bridge priority is only the first part of the bridge ID; the full bridge ID (priority + MAC address) is compared. If priorities are equal, the MAC address is used as a tiebreaker.
★ When this WOULD be the correct answer
In a different question that asks specifically for the criteria used to determine the root bridge when only considering priority values, option B could be correct if the question states that all bridges have the same MAC address, thus making the lowest bridge priority the deciding factor.
Why candidates choose this
It is tempting because priority is the most significant field in the bridge ID, leading some to think it alone determines the root bridge, but the MAC address is also part of the comparison.
✗Highest interface bandwidthWrong answer — click to see why▾
Why this is wrong here
Interface bandwidth is used to calculate path cost, which influences port roles (root port, designated port) but does not affect root bridge election. Root bridge election is based solely on bridge ID.
★ When this WOULD be the correct answer
In a different question that asks about factors influencing network performance or path selection in a Layer 2 network, the highest interface bandwidth could be relevant. For example, a question might ask which attribute affects the best path selection for data traffic in a network topology.
Why candidates choose this
Students may think higher bandwidth is better and assume it influences root bridge selection, confusing path cost metrics with the bridge ID comparison.
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
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
Root bridge
The root bridge is the central reference point in a Spanning Tree Protocol (STP) network, serving as the logical root of the spanning tree topology.
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
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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.