MTU black hole on cellular path

How it presents

Symptom

MTU black hole on cellular path — large OCPP frames fail while tiny pings seem fine; Boot/FW/MeterValues flake on LTE. protocol field note: keep evidence scrubbed and prove the ranked cause before module RMA on NET-C05.

Root causes (by observed frequency)

1. #1 Path MTU discovery blocked; large packets black-holed

   Classic LTE issue

2. #2 Wrong modem MTU/MRU for the carrier
3. #3 VPN overlay forcing undersized MTU
4. #4 Fragmentation firewall drops

Diagnostic procedure

1. 1. Test small vs large payload reachability to CSMS

   Expected: Large fails

   Tools: IT/modem tools

2. 2. Lower modem/charger MTU per carrier OEM note

   Expected: Large frames pass

   Tools: modem UI

3. 3. Ensure ICMP PTB not blocked on path

   Expected: PMTUD works

   Tools: IT

4. 4. Retest BootNotification and MeterValues under load

   Expected: Stable

   Tools: CSMS

5. 5. Document working MTU on the site sheet

   Tools: commissioning

The fix

MTU black holes look like flaky OCPP — fix MTU before modem crates.

1. Clamp MTU to a carrier-safe value rather than swapping modems blindly.
2. Unblock ICMP need-to-fragment messages where policy allows.
3. Re-test large OCPP messages after MTU change. Document readings and scrubbed photos for NET-C05 before closing the ticket.
4. Avoid stacking VPN overlays that shrink MTU further without clamp.
5. Only RMA modem if MTU-correct path still fails. Document readings and scrubbed photos for NET-C05 before closing the ticket.