The GPON Stick Experiment: One Less Box, One Very Hot Dongle

I tried to replace my ISP's ONU box with a Huawei MA5671A SFP stick in the MikroTik. It worked. Then it hit 80°C.

The Dream: Killing the ONU Box

Every home-lab person eventually stares at their ISP's optical modem — that little plastic box between the fiber and the router — and thinks: you don't deserve a power outlet.

Mine is a Viettel-issued ONU that does exactly one job: convert light into Ethernet. It's one more box, one more wall wart, one more thing to fail. And there's a well-known cure: a GPON SFP stick — an entire fiber modem crammed into a thumb-sized transceiver that slots straight into a router's SFP cage.

My MikroTik hEX S has exactly one SFP port, sitting there unused. The math seemed obvious.

The Setup

I got my hands on the community favorite: the Huawei MA5671A, a GPON stick that famously runs a tiny OpenWrt image.

The plan:

  1. Flash / verify OpenWrt SFP 7.5.3 on the stick — it's a full little Linux box, reachable over SSH once it's seated in the cage.
  2. Configure the GPON parameters in /etc/config/gpon and clone the serial number from the ISP's ONU so Viettel's OLT would accept the stick as if it were the original box.
  3. Tag VLAN 35 (Viettel's internet service VLAN) and let the MikroTik dial PPPoE directly.

To my genuine surprise, the hard part went smoothly. The OLT accepted the cloned serial, the stick synced, and packets flowed.

The Part That Was Actually Worth It

Before the stick even arrived, I'd already done the prerequisite work: switching the connection from the ONU's router mode to bridge mode.

That change alone was the real upgrade:

  • The MikroTik now dials PPPoE itself on a vlan35-viettel interface (pppoe-out1).
  • Single NAT instead of the double-NAT sandwich the ISP box loved to serve.
  • The public IP lands directly on my router, where my firewall rules and port-forwards (in-interface-list=WAN) actually live.

Topology after the change:

Internet → Viettel OLT (fiber, bridge)
        → MikroTik pppoe-out1 (PPPoE + NAT)
        → 192.168.88.0/24 (the lab)

If you take one thing from this post: bridge mode + PPPoE on your own router is 90% of the benefit, with 0% of the drama that follows.

The Drama That Follows

One day after the stick went in, I checked its temperature sensor.

80°C. Kissing 79°C on a good minute.

GPON SFP sticks are notorious for this — you're running an entire SoC, optics, and a Linux userland inside a metal sliver with zero airflow, seated in a passively-cooled router that was never designed to host a space heater. Sustained 80°C in a tropical Hanoi apartment, inside a cabinet, in June? That's not an experiment anymore; that's a countdown.

I gave it a day of monitoring, thought about heatsinks, thought about drilling the case, thought about the phrase "single point of failure for the entire household internet"... and pulled the plug.

The Retreat

On June 8 the stick came out, and the trusty plastic ONU box went back on duty — still in bridge mode, still feeding the MikroTik's PPPoE session, still single-NAT. The MikroTik config was reverted to its baseline, and the ONU box quietly resumed its job at 192.168.254.254, humming along at a temperature that doesn't require a fire extinguisher nearby.

Final scorecard:

  • ✅ Bridge mode + PPPoE on the MikroTik — permanent keeper.
  • ✅ Learned the entire GPON auth dance (serials, VLANs, OMCI) — knowledge is free.
  • ❌ One less box on the shelf — the box stays. It won.
  • 🔥 The MA5671A — back in the drawer, waiting for a router with active cooling. Or winter.

Some experiments succeed by teaching you exactly where the line is. This one drew it at 80 degrees.

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