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200-901 Practice Question: A company has two Cisco Catalyst switches, SW1…
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?
⚠ Common exam trap
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.
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
✓
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.
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
SW1's trunk permits only VLAN 10, so VLAN 20 frames arriving from SW2 are dropped before reaching SW1's VLAN 20 devices. SW2's allowed list is irrelevant; the missing VLAN 20 on SW1's trunk is the actual cause of the connectivity failure.
- ✗
SW1 is the root bridge for VLAN 20, causing traffic to be blocked.
Why it's wrong here
Root bridge election does not block a VLAN; SW1 being root for VLAN 20 would still forward traffic across the trunk. It is tempting because spanning tree topology affects paths, and would matter if a blocked port or loop existed rather than a missing allowed VLAN.
- ✗
The trunk link between SW1 and SW2 is down.
Why it's wrong here
A down trunk would also break VLAN 10 connectivity, yet SW1 users still reach VLAN 10 resources on SW2, so the link is forwarding. It is tempting because a failed trunk does block inter-switch VLANs, and would be the cause if all VLANs, not just VLAN 20, lost connectivity.
- ✗
The server in VLAN 20 has an incorrect IP address configuration.
Why it's wrong here
The server already communicates with other VLAN 20 devices on SW2, which an incorrect IP configuration would prevent. It is tempting because misconfigured addressing commonly breaks reachability, and would be the cause if the server failed to reach its own VLAN 20 peers as well.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-901 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 200-901 exam.