netplan VLAN interface fails to come up

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Configured VLAN interface is absent after boot on Ubuntu 26.04 host.

Environment & Reproduction

ip link does not show expected vlan subinterface and routed traffic for that segment fails.

Root Cause Analysis

Validate netplan VLAN stanza, parent NIC name, and switch trunk tagging configuration.

Quick Triage

VLAN ID typo, wrong link association, or upstream switch port not trunked correctly.

Step-by-Step Diagnosis

Correct netplan VLAN mapping and apply config, then ensure switch allows the specified VLAN tag.

Solution – Primary Fix

Confirm vlan interface appears with correct IP and successful ping to gateway.

Solution – Alternative Approaches

Maintain shared network inventory that maps host links to VLAN IDs and trunk policy.

Verification & Acceptance Criteria

Revert to previous netplan file and disable broken VLAN stanza while restoring connectivity.

Rollback Plan

Generate netplan VLAN definitions from central source-of-truth network data.

Prevention & Hardening

netplan try; ip -d link show; bridge vlan show; journalctl -u systemd-networkd -b

Related Errors & Cross-Refs

Include host netplan YAML and switch port config excerpt for joint network review.

Related tutorial: View the step-by-step tutorial for Ubuntu 26.04 LTS.

View all Ubuntu 26.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

NIC offload settings rarely cause VLAN absence but can affect performance once link is active.