The LFP (link fault pass through) means the link fault on the one side (local side) media converter will be passed to the media converter on the other side (remote side). 

For example, the media converter on side A (local side) has the TP link loss, the media converter will disconnect the link of transmit on fiber. The media converter on the side B (remote side) will know there is the linkage error and also disconnect it TP link. 

 The LFP function can immediately alarm network administrators the problem of the link media and provide efficient solution to monitor the network, which can minimize the loss caused by the link problem. 

The LFP function include the LLCF(link loss carry forward) and LLR (Link loss return) function.

LLCF (Link Loss Carry Forward) 

When the TP line of the media converter loss the link, the media converter抯 fiber will disconnect the link of transmit, so that the other side media converter will know that there is a linkage error on this end.  

When the media converter抯 fiber line loss the link, the converter抯 TP will disconnected, and the media converter on the other side will know that there is linkage problem exist.

LLR (Link Loss Return)

LLR(Link loss return) means when a device (Ethernet switch or router) connected to the media converter and the fiber line loss the link, the media converter抯 fiber will disconnect the link of the transmit.

When link fault pass through function is enabled, link status on TX port will inform the FX port of the same device and vice versa. From the link fault pass through procedure illustrates in the figure below, if link fail happens on TX port (1), the local FX port sends non-idle pattern to notice the remote FX port (2). The remote FX port then forces its TX port to link failed after receiving the non-idle pattern (4). In other words, this mechanism will alert the link fault status of local TX port to the remote converter抯 TX port, and the link status of the remote TX port will become off. Link status LED will also be off for both converter.