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Incident Replay: Jumbo Frames Partial Deployment

Setup

  • System context: Storage network where jumbo frames (MTU 9000) were enabled on servers but only partially on the switch infrastructure. Large I/O operations fail while small ones succeed.
  • Time: Wednesday 22:00 UTC
  • Your role: Storage/network engineer

Round 1: Alert Fires

[Pressure cue: "Storage team reports iSCSI connections to the SAN are dropping during large block writes. Small reads work fine. 'It worked before we enabled jumbo frames.'"]

What you see: NFS/iSCSI connections drop with 'Connection reset' during large transfers. ping -s 8972 -M do stor-01 (test jumbo frame) fails with 'Message too long'. Small pings work.

Choose your action: - A) Disable jumbo frames on the servers to restore service - B) Test MTU end-to-end to find where jumbo frames are not supported - C) Check for IP fragmentation issues - D) Update the iSCSI initiator configuration

If you chose A:

[Result: Rolling back MTU to 1500 restores connectivity but loses the performance benefit jumbo frames were supposed to provide.]

[Result: ping -s 8972 -M do to each hop reveals the inter-switch link between switch A and switch B has MTU 1500. Servers and their local switches support 9000, but the core uplink does not. Proceed to Round 2.]

If you chose C:

[Result: DF (Don't Fragment) bit is set on iSCSI traffic. Fragmentation is not happening — packets are being dropped. Partial clue.]

If you chose D:

[Result: iSCSI config is correct. The network path is the problem.]

Round 2: First Triage Data

[Pressure cue: "Problem scoped. Apply the fix."]

What you see: Root cause from Round 1 narrows the investigation. You need to apply the correct fix and verify.

Choose your action: - A) Apply the quick targeted fix - B) Apply the comprehensive fix with verification - C) Apply a workaround while planning the proper fix - D) Escalate to a specialist team

If you chose A:

[Result: Quick fix resolves the immediate issue but may not be durable. Proceed cautiously.]

[Result: Comprehensive fix applied with verification steps. Issue resolved. Proceed to Round 3.]

If you chose C:

[Result: Workaround buys time but the root cause remains. Acceptable short-term.]

If you chose D:

[Result: Specialist is unavailable or adds delay. Try the fix yourself first.]

Round 3: Root Cause Identification

[Pressure cue: "Fix applied. Document root cause and prevention."]

What you see: Root cause is confirmed. Process or configuration gap that allowed this to happen is identified.

Choose your action: - A) Fix the specific instance only - B) Fix the instance and add monitoring - C) Fix the instance, add monitoring, and update procedures - D) Comprehensive: fix + monitor + procedure + automation

[Result: All layers addressed. Immediate fix, detection, process, and automation. Proceed to Round 4.]

If you chose A:

[Result: Fixes this case but the same mistake can recur.]

If you chose B:

[Result: Better detection next time but does not prevent recurrence.]

If you chose C:

[Result: Good coverage but automation reduces human error further.]

Round 4: Remediation

[Pressure cue: "Service restored. Verify and close."]

Actions: 1. Verify the service is functioning correctly 2. Verify monitoring detects the fix 3. Update runbooks and procedures 4. Schedule follow-up actions (automation, infrastructure changes) 5. Close the incident with a post-mortem

Damage Report

  • Total downtime: Varies based on path taken
  • Blast radius: Affected service and dependent systems
  • Optimal resolution time: 15 minutes
  • If every wrong choice was made: 90 minutes + additional damage

Cross-References