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 10.11 provides some examples of link-light indicators functioning on a typical hub or switch.

Table 10.11. Example Link-Light Indicator LED States for a Network Hub or Switch

LED State

Meaning

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 10.11.

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.

If a connection LED on a hub is not lit, all the physical connections are correct, and the connected system is powered on, you might have a faulty patch cable.


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.

LEDs on NICs and Other Devices

In addition to hubs and switches, most other networking devices have LEDs that provide a variety of information. Most NICs have at least one LED that indicates whether there is a link between the system and the network into which it is plugged. The link light operates at a physical level; in other words, it should be lit when the PC is on, regardless of whether the networking software is loaded, the network configuration is correct, or the user is logged on to the network. In addition to the link light LED, many NICs have additional lights to indicate the speed at which the network connection is established and/or when there is network activity on the link.

LEDs are also included on cable modems and DSL modems, which are commonly used in small or home office implementations for Internet connectivity. The number of LEDs and their functionality depends on the device. For example, one cable modem might have four LEDs: one indicating that the modem is online, a Send indicator, a Receive indicator, and one labeled Message. In contrast, a DSL modem might have six LEDs. One shows that the device is powered, and one flashes to indicate that the device is operating normally. Then there is a link light for both the local network and the DSL connection, and another LED for each interface that flashes to indicate activity on those links.

The usefulness of LEDs in troubleshooting scenarios cannot be overstated. LEDs provide an instant, visual indicator about the state of a network link. In some cases, as with collision lights, they can even alert you to problems on the network. Understanding how to interpret information provided by LEDs is important for the real world and for the Network+ exam.

Imagine a scenario in which a user who is working at workstation A calls and tells you she is unable to access the Internet. The Internet connection could be down, but by connecting to the Internet yourself, you determine that it is working correctly; therefore, it is safe to assume that the problem is at the user's end rather than with the Internet connectivity. Next, you decide to visit the user's workstation to see whether you can ping the Internet router. Before you begin the ping test, you look at the back of the system and see that the link LED on the NIC is not lit. At this point, you can be fairly sure that the ping test will not work because without the link light, there is no connectivity between the NIC and the switch.

Now you have narrowed the problem to one of a few sources. Either the NIC or the cable is faulty, the switch to which the user is connected is not functioning, or the port on the switch to which the user is connected is faulty.

The easiest way to test whether the cable is the problem is to borrow a known working cable from workstation B or C and swap it with the cable connecting workstation A to the hub, switch, or wall port. When you try this, if the link light does not come on, you can deduce that the NIC is faulty. If the light does come on, you can deduce that either the port on the switch or a cable is faulty. The next step is to swap the cable out or try the original cable in another switch port.

Expect to be asked to identify the purposes of link lights on the Network+ exam. You might be presented with diagrams and asked how you would use LEDs in the troubleshooting process.


Whatever the actual problem, link lights play an important role in the troubleshooting process. They give you an easy method of seeing what steps do and don't work.



    Network+ Exam Cram 2
    Network+ Exam Cram 2
    ISBN: 078974905X
    EAN: N/A
    Year: 2003
    Pages: 194

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