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350-401 Practice Question: Consider the following configuration on a Cisco…
Consider the following configuration on a Cisco IOS-XE switch:
interface GigabitEthernet0/3 switchport trunk encapsulation dot1q switchport mode trunk switchport trunk native vlan 1
What is the effect of this configuration?
⚠ Common exam trap
Cisco often tests the misconception that a trunk port drops untagged frames or that the native VLAN is only for management traffic, when in fact untagged frames are always associated with the native VLAN on an 802.1Q trunk.
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 interface will operate as an 802.1Q trunk, and untagged frames will be associated with VLAN 1.
The configuration sets the interface to 802.1Q trunking mode and explicitly defines VLAN 1 as the native VLAN. On an 802.1Q trunk, the native VLAN is the VLAN to which untagged frames are assigned when received on the trunk port. Since VLAN 1 is the default native VLAN and is explicitly configured here, untagged frames will be associated with VLAN 1, allowing them to traverse the trunk without an 802.1Q tag.
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 interface will operate as an 802.1Q trunk, and untagged frames will be associated with VLAN 1.
Why this is correct
With 'switchport mode trunk' and 'switchport trunk encapsulation dot1q', the interface inserts a 4-byte 802.1Q tag for frames on all VLANs except the native VLAN. Because the default native VLAN is VLAN 1, any untagged frame received on the trunk is classified as belonging to VLAN 1 and forwarded accordingly. This is the standard behavior for a dot1q trunk unless the native VLAN is explicitly changed with 'switchport trunk native vlan'.
- ✗
The interface will operate as an ISL trunk.
Why it's wrong here
ISL is a Cisco-proprietary encapsulation that prepends a 26-byte header and appends a 4-byte FCS, whereas 802.1Q is an IEEE standard that inserts a 4-byte tag into the frame. The configuration explicitly sets the trunk encapsulation to dot1q, and modern Cisco switches no longer support ISL at all, so the interface cannot operate as an ISL trunk. ISL also has no native VLAN concept, making it fundamentally incompatible with the untagged-frame handling described in the correct answer.
- ✗
The interface will drop all untagged frames.
Why it's wrong here
On an 802.1Q trunk, untagged frames are not discarded; they are always assigned to the native VLAN, which defaults to VLAN 1. Frames in the native VLAN are transmitted without an 802.1Q tag, and received untagged frames are accepted as belonging to that native VLAN. Dropping would happen only if 'switchport trunk native vlan tag' is configured or if the native VLAN values are mismatched on the two ends, neither of which is present here.
- ✗
The interface will only forward traffic for VLAN 1.
Why it's wrong here
By default, an 802.1Q trunk forwards traffic for all active VLANs, not only VLAN 1. The native VLAN is simply the VLAN whose frames travel untagged, while every other VLAN's frames are tagged with the corresponding VLAN ID. To restrict the trunk to only VLAN 1, the configuration must include an explicit 'switchport trunk allowed vlan 1' command, which is not specified in this scenario.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.