Frame Loss Ratio (FLR) is a characterization of the portion of lost Service Frames between the ingress External Interface (EI) and the egress External Interface (EI). Frame Loss Ratio is expressed as a percentage.

FLR is a performance metric for a specific CoS ID of an EVC or OVC.

One-Way FLR

One-way Frame Loss Ratio (FLR) for a given EVC and for a specific CoS ID measures the fraction of compliant frames dropped within the Carrier Ethernet Network (CEN).

Only the frames meeting all the following conditions are counted for FLR performance of an EVC:-

  • Frames that entered the CEN within time interval T
  • Frames declared green by the appropriate ingress BWP
  • Frames which have been forwarded to the egress UNI (not dropped due to conditional delivery or L2CP handling rules)


Frame Loss Ratio = (Number of frames lost or received errored) / (Number of compliant frames sent)


The computation method above is applicable for a single ordered pair of UNIs. For a set  S  of ordered pairs, the FLR is computed as the maximum of the individual measurements for each pair.

Clearly, computing the FLR requires measuring the frame loss between each pair of MEPs. ITU-T Y.1731 defines two procedures for measuring frame loss.


FLR is given from UNI A to UNI B, and can be different for each direction. The formal definition of one-way FLR is provided in Section 8.8.3 MEF 10.3.    

FLR Calculation Example



Related and Further Reading


 Section 8.8.3 MEF 10.3 |