BDCOM ONU Offline: Deregistered & LOS Troubleshooting Guide

A customer says the internet is down. On your BDCOM OLT, their ONU no longer shows auto-configured. Instead it says deregistered, with a reason next to it. This guide shows how to read that reason and work out whether the fault is power, fiber, a blocked ONU, or a registration problem.

The examples use an EPON OLT such as the P3310 series. Other BDCOM EPON models use the same commands. If you run BDCOM GPON, see Configure GPON BDCOM OLT and ONU, because GPON commands differ.

What “Offline” Looks Like on a BDCOM OLT

BDCOM EPON OLTs don’t print “LOS” the way some other vendors do. They show the ONU’s registration state and the reason it dropped. Loss of light appears as a deregistered ONU, usually with the reason llid-admin-down, or power-off if the ONU lost power.

So the first job is always the same: read the status, then follow the reason.

Step 1: Read the ONU Status

enable
show epon onu-information interface EPON0/1

Replace EPON0/1 with the PON port you are checking. You will see one line per ONU, for example:

EPON0/1:8  BDCM 6014 0055.b11e.5534 N/A static deregistered   power-off
EPON0/1:10 XPON XPON a28b.0828.7a90 N/A static deregistered   llid-admin-down
EPON0/1:14 XPON XPON a28b.0828.7260 N/A static auto-configured N/A

Read the last two columns:

auto-configured: the ONU is registered and healthy. Look at the customer’s router or Wi-Fi instead.
deregistered + power-off: the ONU lost power. Go to Step 2.
deregistered + llid-admin-down: the ONU can’t register. Go to Steps 3 to 5.

You can also list ONUs by state with show epon active-onu and show epon inactive-onu.

Step 2: Power-Off

power-off means the ONU powered down and told the OLT before it went. The OLT and the fiber are working. Ask the customer to check the power adapter and the wall socket. A failed adapter is one of the most common causes of a “down” customer.

Step 3: Look at the Pattern

Before you test anything, look at which ONUs are affected:

One ONU only: suspect that customer’s drop cable, connector or ONU.
Several neighbouring ONUs at once: suspect something they share, such as a splitter, a splice or the feeder.
Every ONU on the PON port: suspect the feeder fiber, the OLT port or its optical module.

This pattern check saves hours. Four consecutive ONUs going llid-admin-down together is almost never four separate faults. Our LLID Admin Down case study shows a real example, where one burnt splice took down four ONUs.

Step 4: Check Optical Power

Weak light is the most common reason for an ONU to fall off the OLT. Check both directions.

Light the ONU receives from the OLT (ONU RX):

show epon interface ePON 0/1:5 onu ctc optical-transceiver-diagnosis

Light the OLT receives from the ONUs (OLT RX):

show epon optical-transceiver-diagnosis interface EPON 0/1

Replace 0/1:5 with your port and ONU number. Compare the affected ONU against healthy ones on the same port. Exact limits depend on your optical module, so check its datasheet. As a rough guide for typical EPON optics, a healthy ONU receives somewhere around -8 dBm to -27 dBm. Much weaker than that means too much loss. Much stronger can overload the receiver on a short link.

If the readings are poor, calculate whether your design leaves enough margin with our Fiber Link Loss Calculator.

Step 5: Check the Blacklist

If the light levels look fine but the ONU is still llid-admin-down, someone may have blocked it. Check the blacklist on the port:

interface EPON0/1
show epon onu-blacklist

If the ONU’s MAC address is listed, it was blocked on purpose. Remove it:

no epon onu-blacklist mac a28b.0828.70f0

More on blocking in ONU Block on BDCOM OLT.

Step 6: Check the Registration (MAC Binding)

BDCOM EPON binds each ONU by its MAC address. If you replaced a faulty ONU, the new one has a different MAC, so the old binding no longer matches and the new unit won’t register.

Remove the old binding and add the new one:

enable
config
interface epon 0/1
no epon bind-onu mac OLD.MAC.ADDR
epon bind-onu mac NEW.MAC.ADDR
exit
write all

Replace the MAC addresses with your own. Full steps are in How to Register and Remove ONT in BDCOM OLT. Run write all so the change survives a reboot.

Step 7: Isolate the Fault on the Fiber

If the optical readings are poor, work from the customer back toward the OLT, and change one thing at a time.

Clean the connector. Dust on an SC connector is a very common cause. Use a one-click cleaner or a lint-free wipe with isopropyl alcohol.
Inspect the drop cable. Look for sharp bends, staples, crushed sections or a cable pulled tight at the wall.
Test another splitter port. If the ONU registers on a spare port, the original port or its patch cord is bad.
Check splices and joint closures. A degraded splice shows up as light levels that worsen over days, so check any monitoring history you have.
Check the feeder. If many ONUs on the PON are down, look for a cut or damaged feeder.
Try another PON port. If the whole group returns on a spare OLT port, replace the original port’s optical module.

Step 8: Reboot Only If the ONU Is Online

A reboot can’t fix a deregistered ONU, because the OLT can’t send commands to an ONU that isn’t registered. Use it only on an ONU that is online but misbehaving:

enable
epon reboot onu interface epon 0/1:1

Press y when asked to confirm. More in BDCOM How to Reboot ONU from OLT CLI.

Quick Troubleshooting Table

What you see	Most likely cause	First action
One ONU, deregistered, power-off	No power at the customer	Check adapter and socket
One ONU, llid-admin-down, weak light	Drop cable or connector	Clean connector, inspect cable
Several neighbours, llid-admin-down	Shared splitter or splice	Inspect splitter and splices
Whole PON port down	Feeder or OLT module	Test feeder, try another port
llid-admin-down, light is normal	Blacklist rule	Check and remove the rule
Replaced ONU won’t register	Old MAC still bound	Rebind with the new MAC

Prevent It Next Time

Record each ONU’s optical power at installation, so you can spot slow degradation.
Monitor optical levels with a tool such as Zabbix and alert on drops.
Label splitter ports with the customer they serve.
Keep spare patch cords and pre-tested ONUs.
Run write all after every change.

FAQ

What does deregistered mean on a BDCOM OLT?

The ONU is not currently registered with the OLT. The reason beside it, such as power-off or llid-admin-down, tells you why.

What does llid-admin-down mean?

The ONU can’t register. The usual causes are poor optical signal, a blacklist rule, or a failed fiber component.

Does a BDCOM OLT show LOS?

BDCOM EPON OLTs usually show a deregistered state with a reason rather than a LOS flag. Loss of light normally appears as llid-admin-down.

Why won’t my replacement ONU register on a BDCOM OLT?

BDCOM binds ONUs by MAC address. Remove the old binding with no epon bind-onu mac, add the new MAC with epon bind-onu mac, then run write all.

Can I fix a deregistered ONU by rebooting it from the OLT?

No. The OLT can’t send a reboot to an ONU that isn’t registered. Fix the cause first.

Conclusion

Read the status and reason first, look at which ONUs are affected, then check optical power, the blacklist and the MAC binding in that order. Most faults end up being a fiber, connector or power problem. For more commands, see BDCOM OLT All Commands, the BDCOM OLT FAQ and Command Reference and the BDCOM OLT Complete Setup Guide.