FreeBSD 15 – lagg failover interface flaps and causes intermittent packet loss

📖 ~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

Network sessions drop briefly at random intervals due to active member churn in lagg.

Environment & Reproduction

Detected on multi-NIC hosts after switch policy changes or duplex negotiation drift.

ifconfig lagg0
ifconfig -m 
netstat -i

Root Cause Analysis

Link state or carrier transitions force repeated failover between lagg members.

Quick Triage

Collect physical link counters and dmesg events for each member interface.

dmesg -a | egrep -i 'link state changed|lagg' | tail -n 80
ifconfig 

Step-by-Step Diagnosis

Correlate switch logs, negotiated speed/duplex, and watchdog events.

sysctl dev..0 | head -n 80
tcpdump -ni lagg0 -c 80
route -n get default

Solution – Primary Fix

Normalize member media settings and verify lagg protocol alignment with switch side.

ifconfig  media autoselect
ifconfig  media autoselect
service netif restart

Solution – Alternative Approaches

Switch from failover to lacp when infrastructure supports stable aggregation.

sysrc ifconfig_lagg0='laggproto lacp laggport  laggport  inet /'
service netif restart

Verification & Acceptance Criteria

No additional flap events and packet loss remains within baseline thresholds.

ifconfig lagg0
ping -c 100 
netstat -I lagg0 -w 1

Rollback Plan

Return lagg protocol and member list to previous stable profile.

cp /root/backup/rc.conf /etc/rc.conf
service netif restart

Prevention & Hardening

Enable continuous interface flap alerting and record switch-port parity checks.

Related Errors & Cross-Refs

Can resemble upstream router instability or ARP cache churn.

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

if_lagg documentation and FreeBSD network performance tuning guide.