Question 1,735 of 1,389
CCNA Switching and Network Access Practice Question
A network engineer notices that after adding a new switch to the network, a different switch unexpectedly becomes the STP root bridge, disrupting all VLANs. The new switch has the default priority (32768) but has a lower MAC address than all existing switches. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that a lower MAC address alone can cause a root bridge change, but the trap here is that candidates forget the bridge priority is compared first, so a switch with default priority cannot outrank an existing root with a manually lowered priority.
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
✓
The existing root bridge has a bridge priority lower than the default value of 32768
The existing root bridge has a bridge priority lower than the default value of 32768, so even though the new switch has the default priority, its lower MAC address does not make it the root. STP elects the root bridge based on the lowest bridge ID (priority + MAC address). Since the existing root already has a lower priority, it remains the root. The new switch's lower MAC address only becomes a tiebreaker when priorities are equal, which is not the case here.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The new switch is running PVST+ while the existing switches use Rapid PVST+
Why it's wrong here
PVST+ and Rapid PVST+ interoperate using 802.1D BPDUs on native VLANs and per VLAN. Root election is still based on bridge priority and MAC address. A protocol mismatch does not cause a different switch with a higher MAC to become the root.
- ✗
Root Guard is enabled on the new switch’s uplink ports facing the existing root
Why it's wrong here
Root Guard on a port causes the port to become root-inconsistent if superior BPDUs are received. If the new switch has a lower MAC and default priority, the existing root’s BPDUs would be inferior, not superior. Thus Root Guard would not be triggered, and even if it were, it would block the port on the new switch, not make another switch become the root.
- ✓
The existing root bridge has a bridge priority lower than the default value of 32768
Why this is correct
If the existing root bridge’s priority is less than 32768 (e.g., 4096 or 0), its Bridge ID is lower than the new switch’s default 32768 + lower MAC. STP always elects the switch with the lowest Bridge ID as the root bridge. Thus, despite the new switch’s lower MAC, the manually lowered priority keeps the existing switch as root.
- ✗
The new switch was added with a bridge priority of 4096
Why it's wrong here
A bridge priority of 4096 is much lower than the default 32768. If the new switch had a priority of 4096, it would become the root bridge, not a different switch. This contradicts the observed symptom where a different switch becomes root.
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.
✓The existing root bridge has a bridge priority lower than the default value of 32768Correct answer▾
Why this is correct
If the existing root bridge’s priority is less than 32768 (e.g., 4096 or 0), its Bridge ID is lower than the new switch’s default 32768 + lower MAC. STP always elects the switch with the lowest Bridge ID as the root bridge. Thus, despite the new switch’s lower MAC, the manually lowered priority keeps the existing switch as root.
✗The new switch is running PVST+ while the existing switches use Rapid PVST+Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that STP version incompatibility disrupts root election, but both versions use the same BPDU format and root election rules.
✗Root Guard is enabled on the new switch’s uplink ports facing the existing rootWrong answer — click to see why▾
Why this is wrong here
Candidates often associate Root Guard with preventing a switch from becoming the root. However, it does not cause another switch to become root; it just protects the network from unexpected superior BPDUs.
✗The new switch was added with a bridge priority of 4096Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse the symptom and think that a low priority on the new switch causes the problem, but this would make the new switch the root, not another switch.
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 |
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Last reviewed: Jul 4, 2026
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.
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