Fiber dBm transceiver power level at warning

Question

We have cisco nexus 9000 and one of 40G interface showing port transceiver power level at warning when i check on command line i found following.

Question:

what is the normal power level for transceiver in dBm?

what would be the impact if its in warning/alarm level? (currently i am not experiencing issue issue)

# show int e2/11 transceiver details
Ethernet2/11
    transceiver is present
    type is QSFP-40G-SR4
    name is Fiberstore
    part number is QSFP-SR4-40G
    revision is B
    serial number is D87C2016912
    nominal bitrate is 10300 MBit/sec per channel
    Link length supported for 50/125um OM3 fiber is 150 m
    cisco id is 13
    cisco extended id number is 16

Lane Number:1 Network Lane
           SFP Detail Diagnostics Information (internal calibration)
  ----------------------------------------------------------------------------
                Current              Alarms                  Warnings
                Measurement     High        Low         High          Low
  ----------------------------------------------------------------------------
  Temperature   25.79 C        80.00 C    -10.00 C     73.00 C       -3.00 C
  Voltage        3.40 V         3.59 V      3.00 V      3.46 V        3.13 V
  Current        7.03 mA       11.00 mA     2.00 mA    10.00 mA       3.00 mA
  Tx Power      -3.01 dBm       3.99 dBm  -10.60 dBm    0.99 dBm     -7.61 dBm
  Rx Power       1.08 dBm  +    3.99 dBm  -12.51 dBm    0.99 dBm     -9.50 dBm
  Transmit Fault Count = 0
  ----------------------------------------------------------------------------
  Note: ++  high-alarm; +  high-warning; --  low-alarm; -  low-warning

Lane Number:2 Network Lane
           SFP Detail Diagnostics Information (internal calibration)
  ----------------------------------------------------------------------------
                Current              Alarms                  Warnings
                Measurement     High        Low         High          Low
  ----------------------------------------------------------------------------
  Temperature   25.79 C        80.00 C    -10.00 C     73.00 C       -3.00 C
  Voltage        3.40 V         3.59 V      3.00 V      3.46 V        3.13 V
  Current        7.03 mA       11.00 mA     2.00 mA    10.00 mA       3.00 mA
  Tx Power      -2.73 dBm       3.99 dBm  -10.60 dBm    0.99 dBm     -7.61 dBm
  Rx Power       0.08 dBm       3.99 dBm  -12.51 dBm    0.99 dBm     -9.50 dBm
  Transmit Fault Count = 0
  ----------------------------------------------------------------------------
  Note: ++  high-alarm; +  high-warning; --  low-alarm; -  low-warning

Lane Number:3 Network Lane
           SFP Detail Diagnostics Information (internal calibration)
  ----------------------------------------------------------------------------
                Current              Alarms                  Warnings
                Measurement     High        Low         High          Low
  ----------------------------------------------------------------------------
  Temperature   25.79 C        80.00 C    -10.00 C     73.00 C       -3.00 C
  Voltage        3.40 V         3.59 V      3.00 V      3.46 V        3.13 V
  Current        7.03 mA       11.00 mA     2.00 mA    10.00 mA       3.00 mA
  Tx Power      -3.11 dBm       3.99 dBm  -10.60 dBm    0.99 dBm     -7.61 dBm
  Rx Power       1.63 dBm  +    3.99 dBm  -12.51 dBm    0.99 dBm     -9.50 dBm
  Transmit Fault Count = 0
  ----------------------------------------------------------------------------
  Note: ++  high-alarm; +  high-warning; --  low-alarm; -  low-warning

Lane Number:4 Network Lane
           SFP Detail Diagnostics Information (internal calibration)
  ----------------------------------------------------------------------------
                Current              Alarms                  Warnings
                Measurement     High        Low         High          Low
  ----------------------------------------------------------------------------
  Temperature   25.79 C        80.00 C    -10.00 C     73.00 C       -3.00 C
  Voltage        3.40 V         3.59 V      3.00 V      3.46 V        3.13 V
  Current        7.10 mA       11.00 mA     2.00 mA    10.00 mA       3.00 mA
  Tx Power      -3.09 dBm       3.99 dBm  -10.60 dBm    0.99 dBm     -7.61 dBm
  Rx Power       1.56 dBm  +    3.99 dBm  -12.51 dBm    0.99 dBm     -9.50 dBm
  Transmit Fault Count = 0
  ----------------------------------------------------------------------------
  Note: ++  high-alarm; +  high-warning; --  low-alarm; -  low-warning

Answer 1

There's no serious problem, and as you say, you're not seeing any issues.

The danger is if the power level is too high, you will overdrive the receiver, causing distortion which will generate errors. But you're not there yet.

If you want to lower the power, use a longer cable.

Or dirty connectors (just kidding).

Answer 2

According to IEEE 802.3 Clause 86, the low receive power limit for 40GBASE-SR4 is -9.5 dBm, so that should be the alarm level (or slightly higher). The high limit is +2.4 dBm which any receiver should be able to deal with. Some vendors certify their modules differently (for wider range), so you'd need to check the spec sheet for the exact figures.

The warning/alarm level on the low (negative) side tells you that the optical signal is marginal and you're close to (or already) experiencing FCS errors and possibly carrier/link loss. The same goes for the high side where there's also a chance of quickly burning out the receiver. Too bright signals can be lowered by inserting an attenuator (which generally isn't necessary for -S optics).

You should note that DDM figures should not be taken for being 100% precise, some modules are actually quite a bit off. Sometimes, the receiver reports a higher level than the transmitter is sending and so on.

When installing, I take note of the reported power levels on each side and when there's a problem (or when I feel like it) I compare those notes with the current levels. If I see a significant change somewhere there may be a beginning problem.