IBM AIX 7.2 – Common Problem 175 – Diagnosis and Fix

📖 ~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

Application response times spike intermittently. netstat shows elevated retransmit counters even though link layer reports no errors.

Environment & Reproduction

Occurs on LPARs with mismatched MTU or congested upstream switches.

netstat -s | grep -i retrans | head
oslevel -s
uname -a

Root Cause Analysis

Cause is path MTU issues, send/receive buffer saturation, or duplex mismatch on inherited adapters.

Quick Triage

Capture TCP and adapter counters during steady-state and incident windows.

netstat -s | grep -i retrans | head

Step-by-Step Diagnosis

Capture deeper evidence to isolate the failure path.

netstat -s | grep -i retrans | head
entstat -d en0 | head
no -L | grep tcp
ifconfig en0
errpt | head

Solution – Primary Fix

Apply the proven primary fix in a controlled change window.

no -p -o tcp_recvspace=262144
no -p -o tcp_sendspace=262144
no -p -o sb_max=2097152
chdev -l en0 -a mtu=9000
refresh -s inetd

Solution – Alternative Approaches

Use these alternatives when the primary fix is blocked by environmental constraints.

tcpdump -i en0 -c 1000 -w /tmp/cap.pcap
set rfc1323=1
use tcp_nodelay in app

Verification & Acceptance Criteria

Confirm the system meets acceptance criteria after the change.

netstat -s | grep retrans
iperf3 -c partner
entstat -d ent0 | grep -i error

Rollback Plan

Revert cleanly if regressions appear during validation.

no -d tcp_recvspace
chdev -l en0 -a mtu=1500
refresh -s inetd

Prevention & Hardening

Reduce recurrence with monitoring and preventive tuning.

topasrec -L
nmon -c 60
baseline entstat outputs

Related Errors & Cross-Refs

Related to MTU mismatches across VPN, switch port flapping, and NIM transfers.

Related tutorial: View the step-by-step tutorial for aix-7.2.

View all aix-7.2 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

IBM AIX 7.2 Performance Management TCP tuning, no and netstat references.