CCNP Infrastructure Practice Question
A network engineer is deploying a new branch office with a single Cisco Catalyst switch that will connect to the corporate network via a routed uplink. The switch must be able to forward traffic for VLANs 10, 20, and 30 over that single uplink to a router. The router interface is configured with subinterfaces and dot1Q encapsulation. Which switchport configuration should be applied to the uplink port?
⚠ Common exam trap
The trap here is assuming that a trunk port must be configured with dynamic desirable to form a trunk with a router, when routers do not typically participate in DTP and require a static trunk configuration.
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
✓
switchport mode trunk switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20,30
The uplink must be a trunk to carry multiple VLANs to the router's subinterfaces. Configuring the port as a static trunk with 802.1Q encapsulation and limiting allowed VLANs to the ones in use ensures tagged frames reach the correct router subinterfaces. Dynamic trunking or access mode would not reliably support multiple VLANs over a single physical link.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
switchport mode access switchport access vlan 10
Why it's wrong here
An access port carries traffic for only one VLAN and tags nothing. If the uplink were set to access VLAN 10, frames from VLANs 20 and 30 would be dropped or forwarded incorrectly, and the router's subinterfaces for those VLANs would never receive tagged frames. This fails the requirement to carry multiple VLANs over the single link.
- ✗
switchport mode dynamic desirable switchport trunk allowed vlan 10,20,30
Why it's wrong here
Dynamic desirable actively attempts to form a trunk via DTP, but it is not a deterministic configuration for a link to a router, which typically does not run DTP. The port may remain non-trunking, breaking inter-VLAN routing. Explicit static trunking is required for reliability and to avoid negotiation failures with non-switch devices.
- ✓
switchport mode trunk switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20,30
Why this is correct
A trunk port with 802.1Q encapsulation carries multiple VLANs over one physical link, tagging frames with the appropriate VLAN ID. Restricting allowed VLANs to 10, 20, and 30 matches the router's subinterfaces and prevents unnecessary broadcast flooding. This is the standard router-on-a-stick configuration and satisfies the multi-VLAN uplink requirement.
- ✗
switchport mode trunk switchport trunk native vlan 10 switchport trunk allowed vlan 20,30
Why it's wrong here
Setting the native VLAN to 10 means VLAN 10 frames are sent untagged, but the router subinterface for VLAN 10 expects tagged frames unless it is also configured with native VLAN 10. This mismatch would cause VLAN 10 traffic to be misdelivered or dropped. The native VLAN should typically remain the default (VLAN 1) unless explicitly coordinated.
Visual reference
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 350-401 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 350-401 exam.