Troubleshooting 10GBASE-T and the Role of the LOS Pin

The 10GBASE-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 10GBASE-T module sometimes produces a problem with the physical and logical link state. After plugging the SFP-T module into the port, the physical and administrative link state goes to UP even twisted pair cable is not connected to the SFP-T module.
In the first place, this problem causes wrong information about the interface status and may mislead engineers. But in other cases, it could cause unnecessary CPU utilization.
The problem cause is the 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 a link failure. 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 a LOS pin or without this pin. This depends on the switch`s hardware structure. Some switches support this LOS function, and some models don’t. So, for example, if a switch supports the module LOS feature but a transceiver without LOS function is used, then this can cause the link to be in UP state all the time as the SFP-T transceiver is inserted into the port.

Conlcusions

The issue with the 10GBASE-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.