Determining Real-time Optical Power Loss using "show interface transceiver" command
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07-27-2020 07:22 AM
Reading through the various topics here, I haven't seen this question directly addressed, so here goes.
For the sake of discussion, I have two Cisco switches, Switch1 and Switch2. I run the "show interface transceiver" command at both and get the following:
Switch1 Port | Optical Tx Pwr (dBm) | Optical Rx Pwr (dBm) | Switch2 Port | Optical Tx Pwr (dBm) | Optical Rx Pwr (dBm) |
Te1/1/9 | -1.3 | -2.1 | Te1/0/1 | -1.9 | -1.5 |
In this example, Switch1's Te1/1/9 is connected to Switch2's Te1/0/1.
Assuming the measured dBm values provided by each switch's SFP are accurate, can you calculate the real-time loss for the fiber link as follows:
Switch1->Switch2 Loss (dB) = Switch1 TxPwr - Switch2 RxPwr
and
Switch2->Switch1 Loss (dB) = Switch2 TxPwr - Switch1 RxPwr
Of course, this results in a loss of < 0 for Switch1->Switch2.
So ... am I comparing apples to aardvarks here?
v/r
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07-28-2020 06:55 AM
"Of course, this results in a loss of < 0 for Switch1->Switch2."
Eh?
Sw1>Sw2: -1.3dBm - -1.5dBm = -.2dB
Sw2>Sw1: -1.9dBm - -2.1dBm = -.2dB
NB: actual subtractions' value is .2, but loss is always negative
Same loss in both directions - what one might expect, more or less.
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07-29-2020 09:45 AM
@Joseph W. Doherty wrote:
Correct, i.e.: Loss (dB) = TxPwr dBM - Switch1 RxPwr dBM
"Of course, this results in a loss of < 0 for Switch1->Switch2."
Eh?
Sw1>Sw2: -1.3dBm - -1.5dBm = -.2dB
Sw2>Sw1: -1.9dBm - -2.1dBm = -.2dB
NB: actual subtractions' value is .2, but loss is always negative
Same loss in both directions - what one might expect, more or less.
Sorry. I didn't have enough coffee when I wrote that. I have no idea what I was thinking at the time.
Anyway, I wanted to make sure that you could make reasonably accurate loss measurements (sans using an actual calibrated meter) using what the IOS provided.
Thanks!
