🟡 Medium   ⏱ 5–30 min  Last verified: 19 May 2026
Affected versions: SUSE Linux Enterprise Server 15

📖 ~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

Applications hang on large transfers (TLS, NFS, RDP) while small packets succeed.

Environment & Reproduction

Reported on SLES 15 nodes connected through overlay networks or IPSec tunnels.

Root Cause Analysis

Path MTU is smaller than the configured interface MTU, leading to silent drops of fragmented traffic.

Quick Triage

Test path with `ping -M do -s 1472 ` to find the largest unfragmented size.

Step-by-Step Diagnosis

Compare configured MTU on each hop and check `tracepath` output for MTU steps.

Illustrative mockup for sles-15 — network_mtu-fragmentation_terminal
Terminal diagnostics for MTU mismatch breaks large packets across VPN/VLAN — Illustrative mockup — Progressive Robot

Solution – Primary Fix

Set the correct MTU on the interface via NetworkManager or `/etc/sysconfig/network/ifcfg-*`.

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Illustrative mockup for sles-15 — network_mtu-fragmentation_logs
Logs and evidence for MTU mismatch breaks large packets across VPN/VLAN — Illustrative mockup — Progressive Robot

Solution – Alternative Approaches

Enable TCP MSS clamping on the gateway if interface MTU cannot be reduced.

Verification & Acceptance Criteria

Large transfers complete without retransmits and `iperf3` reaches expected throughput.

Rollback Plan

Revert MTU and reboot if changes break other VLAN-tagged services.

Prevention & Hardening

Document MTU values per network and verify on every site to site link.

Pairs with `frag needed and DF set` ICMP messages.

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

View all sles-15 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

SUSE network MTU tuning and tracepath documentation.

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