Interpreting SFP RX power: -40 dBm or below means no light; use an OTDR or light meter to test the fibre
The poster asked how to test fibre cables from a switch. The guidance covers reading optical levels with show interfaces transceiver / show controllers, comparing TX and RX at both ends, and recognising that a reading of -40, -50 or -80 dBm means no signal is being received. Hardware involved: Cisco switches, SFP/SFP+ optics.
- Hardware: Cisco switces
- Transceivers: SFP/SFP+ optics
- Form factor: SFP, SFP+
Symptoms / messages:
RX power -40 dBm or lower
no signal received
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Comments 1
A reading around -30 dBm or lower indicates no light; there is nothing to test the fibre directly except a separate light meter or OTDR. If the span is uder 40 km with matching optics and RX power is very low, suspect a broken strand and check it with an OTDR.