Troubleshooting 100GBASE-T and the Role of the LOS Pin

Introduction

The 100GBASE-T module can sometimes exhibit unexpected behavior with link state detection, leading to incorrect interface status and potential CPU overhead. This issue stems from the Loss of Signal (LOS) pin configuration, which varies between different SFP-T transceivers and switch hardware. Understanding how LOS functionality impacts link state can help engineers prevent misinterpretations and optimize network performance.

Fault description

The 100GBASE-T module sometimes produces a problem with physical and logical link state. After plugging SFP-T module in port the physical and administrative link state goes to UP even twisted pair cable is not connected to SFP-T module.
In the first place this problem causes wrong information about interface status and may mislead engineers. But in other cases it could cause unnecessary CPU utilization.
The problem cause is loss of signal (LOS) pin technical characteristic. The SFP MSA specification defines a pin called LOS to indicate loss of signal to the motherboard. This should be pulled up with a 4.7K to 10K resistor. Pull up voltage between 2.0V and Vcc-T/R+0.3V. When high, this output indicates link fail. Low indicates normal operation. In the low state, the output will be pulled to <0.8V.
There are two additional options to manufacture SFP-T transceivers, with LOS pin or without this pin. This depends on switch`s hardware structure. There are switches which support this LOS function and some models which doesn’t. So for example if switch supports module LOS feature but transceiver without LOS function is used, then this can cause link being in UP state all the time as the SFP-T transceiver is inserted into port.

Conlcusions

 
The issue with the 100GBASE-T module stems from the loss of signal (LOS) pin, leading to incorrect link states and potential CPU overuse. If a switch supports the LOS feature but the transceiver lacks it, the link state may remain “UP” even when no cable is connected. Ensuring compatibility between the switch and transceiver, particularly regarding the LOS pin, can prevent this issue.