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you will be to deal with future problems. If you must be up quickly, make definite plans to return and finish the job, and stick to those plans.
3.1.3.1 Link lights
Perhaps the simplest test is to rely on the network interfaces link lights. Almost all networking equipment now has status lights that show, when lit, that you have functioning connections. If these do
not light when you make a connection, you definitely have a problem somewhere. Keep in mind, however, a lit link light does not necessarily indicate the absence of a problem.
Many devices have additional indicators that give you more information. It is not uncommon to have a transmit light that blinks each time a packet is sent, a receive light that blinks each time a packet is
received, and a collision light that blinks each time the device detects a collision. To get an idea of what is normal, look at the lights on other computers on the same network.
Typically, you would expect to see the receive light blinking intermittently as soon as you connect the device to an active network. Generally, anomalous behavior with the receive light indicates a problem
somewhere else on your network. If it doesnt ever light, you may have a problem with your connection to the network. For example, you could be plugged into a hub that is not connected to the
network. If the light is on all or most of the time, you probably have an overloaded network.
The transmit light should come on whenever you access the network but should remain off otherwise. You may be surprised, however, how often a computer will access the network. It will almost
certainly blink several times when your computer is booted. If in doubt, try some basic networking tasks while watching for activity. If it does not light when you try to access the network, you have
problems on that computer. If it stays lit, you not only have a problem but also are probably flooding the network with packets, thereby causing problems for others on the network as well. You may want
to isolate this machine until the problem is resolved.
In the ideal network, from the users perspective at least, the collision light should remain relatively inactive. However, excessive collision light flashing or even one that remains on most of the time may
not indicate a problem. A collision is a very brief event. If the light only remained on for the length of the event, the flash would be too brief to be seen. Consequently, these lights are designed to remain on
much longer than the actual event. A collision light that remains on doesnt necessarily mean that your network is saturated with collisions. On the other hand, this is something youll want to investigate
further.
For any of the cases in which you have an indication of a network overload, unless your network is completely saturated, you should be able to get some packets through. And you should see similar
problems on every computer on that network segment. If your network is completely saturated, then you may have a malfunctioning device that is continuously transmitting. Usually, this can be easily
located by turning devices off one at a time until the problem suddenly disappears.
If you have an indication of a network overload, you should look at the overall behavior and structure of your network. A good place to start is with netstat as discussed in
Chapter 4 . For a more thorough
discussion of network performance monitoring, turn to Chapter 8
. One last word of warning—you may see anomalous behavior with any of
these lights if your interface is misconfigured or has the wrong driver installed.
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3.1.3.2 Cable testers
