Log in to the switch console to run the privileged EXEC mode of the Cisco switch, use the fiber-ports-optical-transceiver command. The Output Power (mWatt) field in the command output indicates the received power of the optical module, and the Input Power (mWatt) field. Monitoring the optical power of SFP (Small Form-factor Pluggable) modules is a critical step in maintaining stable network links. Even if an interface appears up, degraded Tx/Rx levels can cause intermittent flapping, packet loss, or err-disabled states. Checking optical power helps pinpoint issues. Before you blame the switch or replace the cable, you need to look at the invisible data: the light levels. For network engineers working with fiber optics (SFP, SFP+, QSFP), understanding TX (Transmit) and RX (Receive) signal strength is critical. It is the difference between a stable, high-speed. This guide gives a practical, CLI-focused workflow for checking SFP health and diagnostics on Cisco switches, shows the exact commands you'll use, explains what the numbers mean, and compares OEM (Cisco) vs third-party modules so you can pick the right SFP module supplier for reliability and cost. There are no specific requirements for this document. This guide uses the Moduletek SFP-25G-SR optical module connected to a Cisco C9300 switch as an example.