This 200-901 practice question tests your understanding of network fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
A company has two Cisco Catalyst switches, SW1 and SW2, connected via a trunk link using port GigabitEthernet0/1 on both switches. SW1 is the root bridge for all VLANs spanning tree. VLAN 10 users on SW1 report they can access the internet and resources in VLAN 10 on SW2, but cannot reach a critical server in VLAN 20 connected to SW2. The server in VLAN 20 has a static IP address and can communicate with other VLAN 20 devices on SW2. SW2's configuration for the trunk port includes 'switchport trunk allowed vlan 10,20'. SW1's trunk port configuration is 'switchport trunk allowed vlan 10'. The network administrator has verified that both switches have VLANs 10 and 20 created and that the default gateways are correct. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
SW1's trunk port is not configured to allow VLAN 20.
SW1's trunk port is configured with 'switchport trunk allowed vlan 10', which explicitly permits only VLAN 10 traffic. Since VLAN 20 is not in the allowed list, frames from VLAN 20 (including traffic to the server) are dropped at the trunk egress on SW1. This prevents SW1 hosts in VLAN 10 from reaching the VLAN 20 server on SW2, even though the trunk is up and both VLANs exist on both switches.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
SW1's trunk port is not configured to allow VLAN 20.
Why this is correct
The trunk allowed VLAN list on SW1 only includes VLAN 10, so VLAN 20 traffic is blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
SW1 is the root bridge for VLAN 20, causing traffic to be blocked.
Why it's wrong here
Being root bridge does not block traffic; STP only prevents loops.
The server in VLAN 20 has an incorrect IP address configuration.
Why it's wrong here
The server can communicate with other devices in VLAN 20, so IP configuration is correct.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between VLAN existence on a switch and VLAN permission on a trunk port—candidates assume that if a VLAN is created on both switches, traffic will flow, but the trunk allowed list is the gatekeeper.
Detailed technical explanation
How to think about this question
The 'switchport trunk allowed vlan' command uses a VLAN filter list that is applied at the egress of the trunk port; any VLAN not in the list is silently discarded at Layer 2. This is independent of the VLAN database on the switch—VLAN 20 may exist and be active, but without being permitted on the trunk, no frames for that VLAN traverse the link. In real-world scenarios, this misconfiguration often occurs when administrators update allowed VLANs on one side but forget the other, leading to asymmetric reachability.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: SW1's trunk port is not configured to allow VLAN 20. — SW1's trunk port is configured with 'switchport trunk allowed vlan 10', which explicitly permits only VLAN 10 traffic. Since VLAN 20 is not in the allowed list, frames from VLAN 20 (including traffic to the server) are dropped at the trunk egress on SW1. This prevents SW1 hosts in VLAN 10 from reaching the VLAN 20 server on SW2, even though the trunk is up and both VLANs exist on both switches.
What should I do if I get this 200-901 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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