On any given Cisco router, the LED lights provide quick insight into whether the system or ports are functioning correctly. On a Cisco 2500 series router, each port (with the exception of the console and auxiliary ports) has an associated LED that indicates port activity, in a fashion similar to what you might expect to see on a switch or hub. Another LED, to the right of the auxiliary port, is referred to as the “System OK” LED. When this LED is lit, it indicates that the router is functioning correctly.
In a normal working state, port LEDs will flicker to indicate activity. In cases where the port LED is not active, it suggests either a connectivity issue, or potentially a problem with the port.
Obviously a router needs to be powered. In cases where the router needs to be opened in order to change or upgrade components, not only should it be powered down using the power switch, but the Alternating Current (AC) cable should also be disconnected. Cisco routers are usually also available in Direct Current (DC) versions, since many telecommunications carriers require DC-powered devices. In cases where DC devices need to be powered down, the power switch should be turned off, and then power should be disconnected at the circuit breaker.