FreeBSD 15: CARP state flapping due to VHID skew mismatch

📖 ~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

Failover VIP oscillates, producing packet loss, session resets, and inconsistent service routing.

Environment & Reproduction

Appears after unsynchronized config changes to advskew, passphrase, or interface timing.

Root Cause Analysis

Peers evaluate priority inconsistently, causing repeated ownership contests for shared VHID instances.

Quick Triage

Check CARP status, clock alignment, and matching interface-level parameters on both peers.

Step-by-Step Diagnosis

Capture CARP advertisements and compare node configurations to isolate mismatch fields.

Solution – Primary Fix

Align VHID settings, auth material, and skew policy, then restart CARP interfaces in order.

Solution – Alternative Approaches

Temporarily force one node to persistent backup while maintenance synchronization completes.

Verification & Acceptance Criteria

Role transitions only occur on intentional failover and no spontaneous flapping is observed.

Rollback Plan

Reapply prior CARP configuration set and disable recent policy toggles if instability persists.

Prevention & Hardening

Use configuration templating with drift detection and mandatory peer diff checks.

Related Errors & Cross-Refs

Often linked to pfsync inconsistency and switch multicast filtering anomalies.

Related tutorial: View the step-by-step tutorial for freebsd-15.

View all freebsd-15 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

carp man page, FreeBSD firewall handbook, and high-availability operational guidance.