Interpreting Visual Indicators

One of the easiest ways to spot signs of trouble on a network or with a network component is to look at the devices' LEDs. Many of the devices used in modern networkssuch as hubs, routers, switches, and even NICshave these small indicator lights that let you know what, if anything, is going wrong. The following sections examine some of the common networking devices and what you can learn from their LEDs.

LEDs on Networking Devices

If you have seen a hub or a switch, you have no doubt noticed the LEDs on the front of the device. Each RJ-45 connector has one or two dedicated LEDs. These LEDs are designed to provide the network administrator with a quick idea of the status of a connection or a potential problem. Table 11 provides some examples of link-light indicators functioning on a typical hub or switch.

Table 11 Example Link-Light Indicator LED States for a Network Hub or Switch

LED State


Solid green

A device is connected to the port, but there is no activity on the device.

Blinking green

There is activity on the port. The connected system is sending or receiving data.

No LED lit

There is no detectable link. Either there's a problem with the connection between the device and the hub (such as an unplugged cable), or the remote system is powered down.

Fast continuous blinking for extended periods

This often indicates a fault with the connection, which can commonly be attributed to a faulty NIC.

Blinking amber

There are collisions on the network. A few orange LEDs flashing intermittently are okay, but continuously blinking amber LEDs indicate a problem.

Note that the LEDs' sequencing and meanings vary among the different hub manufacturers and therefore might be different from those listed in Table 11.

In addition to link-light indicators, some hubs and switches have port-speed LEDs that, when lit, indicate the speed at which the connected device is functioning. Some also have LEDs that indicate whether the link is operating in full-duplex mode.

By understanding the function of the lights on networking devices, you can tell at a glance the status of a device and the systems connected to it. You should take the time to familiarize yourself with the indicator lights on the network devices you work with and with their various states.