EXFO FTB-800 Series for FTB-1 - Manual (Page 468)

Test Functions
458 FTB-700G/800 Series
S-OAM Link Trace

RX LTR indicates the count of received LTR frames having their
destination MAC address matching the unit port MAC address, and
VLANs matching the unit port VLANs.

LTR Timeout indicates the count of LTR Timeout event which occurs if
a reply (LTR) from the Peer MEP is not received within 5 seconds.

Invalid LTR indicates the count of LTR frames received with incorrect
MEG/MD Level or Transaction ID.