CCNA Switching and Network Access Practice Question
Exhibit
SW1# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40 SW2# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40,50
Two switches, SW1 and SW2, are connected via a trunk link. Hosts in VLAN 50 on SW1 cannot communicate with hosts in VLAN 50 on SW2, while hosts in other VLANs communicate normally. What is the most likely cause?
⚠ Common exam trap
Be careful to distinguish between total trunk failures and selective VLAN issues. Check the allowed VLAN list on trunk links when only one VLAN is affected.
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
✓
VLAN 50 is not allowed on the trunk from SW1.
The strongest explanation is that VLAN 50 is missing from the allowed VLAN list on one side of the trunk. In practical terms, the trunk is up and carrying other VLANs, so the problem is selective rather than total. When one VLAN is omitted from the allowed list, only that VLAN fails while others continue to work normally. This is a high-value switching troubleshooting pattern because it rewards careful reading of operational output rather than generic trunk theory.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VLAN 50 is not allowed on the trunk from SW1.
Why this is correct
The trunk between SW1 and SW2 has a configured allowed VLAN list that excludes VLAN 50; any frames tagged for that VLAN are dropped at the trunk interface. Because 802.1Q trunks only forward VLANs explicitly permitted in the `switchport trunk allowed vlan` list, the missing entry prevents VLAN 50 traffic from reaching SW2. This would cause clients in VLAN 50 to lose connectivity across the link.
- ✗
The native VLAN must be changed to 50 on both switches.
Why it's wrong here
The native VLAN is merely the VLAN that carries untagged frames on an 802.1Q trunk; it has no bearing on which VLANs are permitted to cross the link. Even if both switches set native VLAN 50, the trunk still would not forward that VLAN unless it is added to the allowed list. A mismatch in native VLAN typically triggers CDP errors or spanning-tree alarms, not the specific symptom of VLAN 50 dropping.
When this WOULD be correct
In a different scenario where the question specifies that both switches are configured with VLAN 50 as the native VLAN and that there is a requirement for untagged traffic to be sent on VLAN 50, this option would be correct. The question could ask about the implications of having mismatched native VLANs causing connectivity issues.
- ✗
The trunk must be changed to an access port.
Why it's wrong here
Replacing the trunk with a single access port would force the interface to carry only one untagged VLAN, removing the multi-VLAN capability needed by the network. An access port would assign the connected device to one access VLAN and strip any 802.1Q tags, so VLAN 50 traffic could still be isolated or dropped if that port is not configured for it. This change would not restore VLAN 50 forwarding; it would simply alter the link's role to a single-VLAN edge connection.
When this WOULD be correct
In a different scenario where the question specifies that a switch is incorrectly configured to use an access port for a connection that should carry multiple VLANs, changing the trunk to an access port could be the correct answer if the goal is to isolate traffic to a single VLAN.
- ✗
The switches must run PPP on the uplink.
Why it's wrong here
PPP (Point-to-Point Protocol) is a layer-2 WAN encapsulation used on serial or dial-up links, not on Ethernet switch trunks. Trunking between Ethernet switches relies on 802.1Q tagging, which is separate from PPP's framing and negotiation. Deploying PPP on a gigabit Ethernet uplink would be both unsupported and irrelevant to the VLAN 50 allowed-list condition.
When this WOULD be correct
In a different scenario where the question specifies that the switches are connected via a serial link requiring PPP for communication, and the configuration is incorrectly set to Ethernet, this option would be correct. The question might ask about the necessary protocol for inter-switch communication in a non-Ethernet environment.
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.
✓VLAN 50 is not allowed on the trunk from SW1.Correct answer▾
Why this is correct
The trunk between SW1 and SW2 has a configured allowed VLAN list that excludes VLAN 50; any frames tagged for that VLAN are dropped at the trunk interface. Because 802.1Q trunks only forward VLANs explicitly permitted in the `switchport trunk allowed vlan` list, the missing entry prevents VLAN 50 traffic from reaching SW2. This would cause clients in VLAN 50 to lose connectivity across the link.
✗The native VLAN must be changed to 50 on both switches.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN is used for untagged traffic on a trunk and does not affect the forwarding of tagged frames for other VLANs. Changing the native VLAN to 50 would not resolve the issue because VLAN 50 frames are still tagged and require inclusion in the allowed VLAN list.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that both switches are configured with VLAN 50 as the native VLAN and that there is a requirement for untagged traffic to be sent on VLAN 50, this option would be correct. The question could ask about the implications of having mismatched native VLANs causing connectivity issues.
Why candidates choose this
Students often confuse the native VLAN with the concept of a 'management VLAN' or think that setting the native VLAN to the desired VLAN will automatically allow that VLAN across the trunk, but the native VLAN only affects untagged frames.
✗The trunk must be changed to an access port.Wrong answer — click to see why▾
Why this is wrong here
An access port can only carry a single VLAN, so changing the trunk to an access port would prevent the link from carrying multiple VLANs, including VLAN 50. The link is intended to carry multiple VLANs, so a trunk is required.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that a switch is incorrectly configured to use an access port for a connection that should carry multiple VLANs, changing the trunk to an access port could be the correct answer if the goal is to isolate traffic to a single VLAN.
Why candidates choose this
Some test-takers might think that if a specific VLAN is not working, converting the link to an access port and assigning it to VLAN 50 could solve the problem, but this would break connectivity for other VLANs and is not a proper solution for inter-switch VLAN communication.
✗The switches must run PPP on the uplink.Wrong answer — click to see why▾
Why this is wrong here
PPP (Point-to-Point Protocol) is a WAN protocol used on serial links, not on Ethernet trunks. Ethernet trunks use 802.1Q or ISL encapsulation, and PPP has no relevance to VLAN trunking issues.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that the switches are connected via a serial link requiring PPP for communication, and the configuration is incorrectly set to Ethernet, this option would be correct. The question might ask about the necessary protocol for inter-switch communication in a non-Ethernet environment.
Why candidates choose this
Students might recall that PPP can be used for authentication or encapsulation on point-to-point links and mistakenly think it could be applied to Ethernet trunks, but PPP is not used in Ethernet switching environments.
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?”
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Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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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.