Question 54 of 2,015

VLANs and Trunking →mediumMatchingObjective-mapped

Quick Answer

The correct answer is 802.1Q, which uses a 4-byte tag containing a 12-bit VLAN ID supporting 0–4095 VLANs, while ISL is a Cisco-proprietary protocol that encapsulates the entire frame without a native VLAN concept. This distinction matters because 802.1Q is the open standard that inserts its tag between the source MAC and EtherType fields, allowing interoperability across vendors, whereas ISL adds a 26-byte header and 4-byte trailer, making it incompatible with non-Cisco hardware. On the ENCOR 350-401 exam, this comparison tests your ability to differentiate proprietary versus standards-based trunking, often appearing in drag-and-drop or multiple-choice questions where a common trap is confusing encapsulation (ISL) with tagging (802.1Q). Remember that 802.1Q is the default on modern switches and supports a native VLAN for untagged traffic, while ISL does not. A quick memory tip: “Q is for Quad-byte tag and Quick standard adoption; ISL is for Isolated Cisco Legacy.”

CCNP VLANs and Trunking Practice Question

This 350-401 practice question tests your understanding of vlans and trunking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.

Drag and drop each trunk encapsulation type on the left to its matching standard or characteristic on the right.

Question 1mediummatching

Open the full VLAN trunking answer →

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

✓

802.1Q: Open standard (IEEE); inserts 4-byte tag; supports native VLAN

802.1Q is an open standard that inserts a 4-byte tag, supports native VLAN, and is the default on modern switches. ISL is Cisco proprietary, encapsulates the entire frame, and does not support native VLAN.

Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.

Common exam traps

Common exam trap: an active trunk can still block the VLAN you need

A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.

Detailed technical explanation

How to think about this question

VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.

KKey Concepts to Remember

Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.

TExam Day Tips

Use show vlan brief to verify access VLANs.
Use show interfaces trunk to verify trunk state and allowed VLANs.
Do not treat every same-VLAN issue as a routing problem.

Key takeaway

A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.

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.

Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.

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FAQ

Questions learners often ask

What does this 350-401 question test?

VLANs and Trunking — This question tests VLANs and Trunking — Access ports place end devices into a single VLAN..

What is the correct answer to this question?

The correct answer is: 802.1Q: Open standard (IEEE); inserts 4-byte tag; supports native VLAN — 802.1Q is an open standard that inserts a 4-byte tag, supports native VLAN, and is the default on modern switches. ISL is Cisco proprietary, encapsulates the entire frame, and does not support native VLAN.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

Access ports place end devices into a single VLAN.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 350-401

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Drag and drop each trunk encapsulation on the left to its matching standard on the right.

medium

✓ P1.ISL: Cisco proprietary, encapsulates entire frame with 30-byte header
✓ P2.802.1Q: IEEE standard, inserts 4-byte tag into frame
✓ P3.802.1Q native VLAN: Untagged frames on trunk port
✓ P4.ISL: Does not support native VLAN concept
✓ P5.802.1Q: Supports up to 4094 VLANs

Why P1: ISL is Cisco proprietary with 30-byte header; 802.1Q is IEEE standard with 4-byte tag; only 802.1Q supports native VLAN; ISL encapsulates entire frame.

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