The first step in diagnosing a network problem is to collect information. This includes collecting information from your users as to the nature of the problems they are having, and it includes collecting data from your network. Your success will depend, in large part, on your efficiency in collecting this information and on the quality of the information you collect. This book is about tools you can use and techniques and strategies to optimize their use. Rather than trying to cover all aspects of troubleshooting, this book focuses on this first crucial step, data collection.
There is an extraordinary variety of tools available for this purpose, and more become available daily. Very capable people are selflessly devoting enormous amounts of time and effort to developing these tools. We all owe a tremendous debt to these individuals. But with the variety of tools available, it is easy to be overwhelmed. Fortunately, while the number of tools is large, data collection need not be overwhelming. A small number of tools can be used to solve most problems. This book centers on a core set of freely available tools, with pointers to additional tools that might be needed in some circumstances.
This first chapter has two goals. Although general troubleshooting is not the focus of the book, it seems worthwhile to quickly review troubleshooting techniques. This review is followed by an examination of troubleshooting from a broader administrative context—using troubleshooting tools in an effective, productive, and responsible manner. This part of the chapter includes a discussion of documentation practices, personnel management and professionalism, legal and ethical concerns, and economic considerations. General troubleshooting is revisited in Chapter 12, once we have discussed available tools. If you are already familiar with these topics, you may want to skim or even skip this chapter.
Troubleshooting is a complex process that is best learned through experience. This section looks briefly at how troubleshooting is done in order to see how these tools fit into the process. But while every problem is different, a key step is collecting information.
Clearly, the best way to approach troubleshooting is to avoid it. If you never have problems, you will have nothing to correct. Sound engineering practices, redundancy, documentation, and training can help. But regardless of how well engineered your system is, things break. You can avoid troubleshooting, but you can’t escape it.
It may seem unnecessary to say, but go for the quick fixes first. As long as you don’t fixate on them, they won’t take long. Often the first thing to try is resetting the system. Many problems can be resolved in this way. Bit rot, cosmic rays, or the alignment of the planets may result in the system entering some strange state from which it can’t exit. If the problem really is a fluke, resetting the system may resolve the problem, and you may never see it again. This may not seem very satisfying, but you can take your satisfaction in going home on time instead.
Keep in mind that there are several different levels in resetting a system. For software, you can simply restart the program, or you may be able to send a signal to the program so that it reloads its initialization file. From your users’ perspective, this is the least disruptive approach. Alternately, you might restart the operating system but without cycling the power, i.e., do a warm reboot. Finally, you might try a cold reboot by cycling the power.
You should be aware, however, that there can be some dangers in resetting a system. For example, it is possible to inadvertently make changes to a system so that it can’t reboot. If you realize you have done this in time, you can correct the problem. Once you have shut down the system, it may be too late. If you don’t have a backup boot disk, you will have to rebuild the system. These are, fortunately, rare circumstances and usually happen only when you have been making major changes to a system.
When making changes to a system, remember that scheduled maintenance may involve restarting a system. You may want to test changes you have made, including their impact on a system reset, prior to such maintenance to ensure that there are no problems. Otherwise, the system may fail when restarted during the scheduled maintenance. If this happens, you will be faced with the difficult task of deciding which of several different changes are causing problems.
Resetting the system is certainly worth trying once. Doing it more than once is a different matter. With some systems, this becomes a way of life. An operating system that doesn’t provide adequate memory protection will frequently become wedged so that rebooting is the only option. Sometimes you may want to limp along resetting the system occasionally rather than dealing with the problem. In a university setting, this might get you through exam week to a time when you can be more relaxed in your efforts to correct the underlying problem. Or, if the system is to be replaced in the near future, the effort may not be justified. Usually, however, when rebooting becomes a way of life, it is time for more decisive action.
Swapping components and reinstalling software is often the next thing to try. If you have the spare components, this can often resolve problems immediately. Even if you don’t have spares, switching components to see if the problem follows the equipment can be a simple first test. Reinstalling software can be much more problematic. This can often result in configuration errors that will worsen problems. The old, installed version of the software can make getting a new, clean installation impossible. But if the install is simple or you have a clear understanding of exactly how to configure the software, this can be a relatively quick fix.
While these approaches often work, they aren’t what we usually think of as troubleshooting. You certainly don’t need the tools described in this book to do them. Once you have exhausted the quick solutions, it is time to get serious. First, you must understand the problem, if possible. Problems that are not understood are usually not fixed, just postponed.
One standard admonition is to ask the question “has anything changed recently?” Overwhelmingly, most problems relate to changes to a working system. If you can temporarily change things back and the problem goes away, you have confirmed your diagnosis.
Admittedly, this may not help with an installation where everything is new. But even a new installation can and should be grown. Pieces can be installed and tested. New pieces of equipment can then be added incrementally. When this approach is taken, the question of what has changed once again makes sense.
Another admonition is to change only one thing at a time and then to test thoroughly after each change. This is certainly good advice when dealing with routine failures. But this approach will not apply if you are dealing with a system failure. (See the upcoming sidebar on system failures.) Also, if you do find something that you know is wrong but fixing it doesn’t fix your problem, do you really want to change it back? In this case, it is often better to make a note of the additional changes you have made and then proceed with your troubleshooting.
A key element to successful debugging is to control the focus of your investigation so that you are really dealing with the problem. You can usually focus better if you can break the problem into pieces. Swapping components, as mentioned previously, is an example of this approach. This technique is known by several names—problem decomposition, divide and conquer, binary search, and so on. This approach is applicable to all kinds of troubleshooting. For example, when your car won’t start, first decide whether you have an electrical or fuel supply problem. Then proceed accordingly. Chapter 12 outlines a series of specific steps you might want to consider.
The best time to prepare for problems is before you have them. It may sound trite, but if you don’t understand the normal behavior of your network, you will not be able to identify anomalous behavior. For the proper management of your system, you must have a clear understanding of the current behavior and performance of your system. If you don’t know the kinds of traffic, the bottlenecks, or the growth patterns for your network, then you will not be able to develop sensible plans. If you don’t know the normal behavior, you will not be able to recognize a problem’s symptoms when you see them. Unless you have made a conscious, aggressive effort to understand your system, you probably don’t understand it. All networks contain surprises, even for the experienced administrator. You only have to look a little harder.
It might seem strange to some that a network administrator would need some of the tools described in this book, and that he wouldn’t already know the details that some of these tools provide. But there are a number of reasons why an administrator may be quite ignorant of his network.
With the rapid growth of the Internet, turnkey systems seem to have grown in popularity. A fundamental assumption of these systems is that they are managed by an inexperienced administrator or an administrator who doesn’t want to be bothered by the details of the system. Documentation is almost always minimal. For example, early versions of Sun Microsystems’ Netra Internet servers, by default, did not install the Unix manpages and came with only a few small manuals. Print services were disabled by default.
This is not a condemnation of turnkey systems. They can be a real blessing to someone who needs to go online quickly, someone who never wants to be bothered by such details, or someone who can outsource the management of her system. But if at some later time she wants to know what her turnkey system is doing, it may be up to her to discover that for herself. This is particularly likely if she ever wants to go beyond the basic services provided by the system or if she starts having problems.
Other nonturnkey systems may be customized, often heavily. Of course, all these changes should be carefully documented. However, an administrator may inherit a poorly documented system. (And, of course, sometimes we do this to ourselves.) If you find yourself in this situation, you will need to discover (or rediscover) your system for yourself.
In many organizations, responsibilities may be highly partitioned. One group may be responsible for infrastructure such as wiring, another for network hardware, and yet another for software. In some environments, particularly universities, networks may be a distributed responsibility. You may have very little control, if any, over what is connected to the network. This isn’t necessarily bad—it’s the way universities work. But rogue systems on your network can have annoying consequences. In this situation, probably the best approach is to talk to the system administrator or user responsible for the system. Often he will be only too happy to discuss his configuration. The implications of what he is doing may have completely escaped him. Developing a good relationship with power users may give you an extra set of eyes on your network. And, it is easier to rely on the system administrator to tell you what he is doing than to repeatedly probe the network to discover changes. But if this fails, as it sometimes does, you may have to resort to collecting the data yourself.
Sometimes there may be some unexpected, unauthorized, or even covert changes to your network. Well-meaning individuals can create problems when they try to help you out by installing equipment themselves. For example, someone might try installing a new computer on the network by copying the network configuration from another machine, including its IP address. At other times, some “volunteer administrator” simply has her own plans for your network.
Finally, almost to a person, network administrators must teach themselves as they go. Consequently, for most administrators, these tools have an educational value as well as an administrative value. They provide a way for administrators to learn more about their networks. For example, protocol analyzers like ethereal provide an excellent way to learn the inner workings of a protocol like TCP/IP. Often, more than one of these reasons may apply. Whatever the reason, it is not unusual to find yourself reading your configuration files and probing your systems.
Troubleshooting does not exist in isolation from network management. How you manage your network will determine in large part how you deal with problems. A proactive approach to management can greatly simplify problem resolution. The remainder of this chapter describes several important management issues. Coming to terms with these issues should, in the long run, make your life easier.
As a new administrator, your first step is to assess your existing resources and begin creating new resources. Software sources, including the tools discussed in this book, are described and listed in Appendix A. Other sources of information are described in Appendix B.
The most important source of information is the local documentation created by you or your predecessor. In a properly maintained network, there should be some kind of log about the network, preferably with sections for each device. In many networks, this will be in an abysmal state. Almost no one likes documenting or thinks he has the time required to do it. It will be full of errors, out of date, and incomplete. Local documentation should always be read with a healthy degree of skepticism. But even incomplete, erroneous documentation, if treated as such, may be of value. There are probably no intentional errors, just careless mistakes and errors of omission. Even flawed documentation can give you some sense of the history of the system. Problems frequently occur due to multiple conflicting changes to a system. Software that may have been only partially removed can have lingering effects. Homegrown documentation may be the quickest way to discover what may have been on the system.
While the creation and maintenance of documentation may once have been someone else’s responsibility, it is now your responsibility. If you are not happy with the current state of your documentation, it is up to you to update it and adopt policies so the next administrator will not be muttering about you the way you are muttering about your predecessors.
There are a couple of sets of standard documentation that, at a minimum, you will always want to keep. One is purchase information, the other a change log. Purchase information includes sales information, licenses, warranties, service contracts, and related information such as serial numbers. An inventory of equipment, software, and documentation can be very helpful. When you unpack a system, you might keep a list of everything you receive and date all documentation and software. (A changeable rubber date stamp and ink pad can help with this last task.) Manufacturers can do a poor job of distinguishing one version of software and its documentation from the next. Dates can be helpful in deciding which version of the documentation applies when you have multiple systems or upgrades. Documentation has a way of ending up in someone’s personal library, never to be seen again, so a list of what you should have can be very helpful at times.
Keep in mind, there are a number of ways software can enter your system other than through purchase orders. Some software comes through CD-ROM subscription services, some comes in over the Internet, some is bundled with the operating system, some comes in on a CD-ROM in the back of a book, some is brought from home, and so forth. Ideally, you should have some mechanism to track software. For example, for downloads from the Internet, be sure to keep a log including a list identifying filenames, dates, and sources.
You should also keep a change log for each major system. Record every significant change or problem you have with the system. Each entry should be dated. Even if some entries no longer seem relevant, you should keep them in your log. For instance, if you have installed and later removed a piece of software on a server, there may be lingering configuration changes that you are not aware of that may come to haunt you years later. This is particularly true if you try to reinstall the program but could even be true for a new program as well.
Beyond these two basic sets of documentation, you can divide the documentation you need to keep into two general categories—configuration documentation and process documentation. Configuration documentation statically describes a system. It assumes that the steps involved in setting up the system are well understood and need no further comments, i.e., that configuration information is sufficient to reconfigure or reconstruct the system. This kind of information can usually be collected at any time. Ironically, for that reason, it can become so easy to put off that it is never done.
Process documentation describes the steps involved in setting up a device, installing software, or resolving a problem. As such, it is best written while you are doing the task. This creates a different set of collection problems. Here the stress from the task at hand often prevents you from documenting the process.
The first question you must ask is what you want to keep. This may depend on the circumstances and which tools you are using. Static configuration information might include lists of IP addresses and Ethernet addresses, network maps, copies of server configuration files, switch configuration settings such as VLAN partitioning by ports, and so on.
When dealing with a single device, the best approach is probably just a simple copy of the configuration. This can be either printed or saved as a disk file. This will be a personal choice based on which you think is easiest to manage. You don’t need to waste time prettying this up, but be sure you label and date it.
When the information spans multiple systems, such as a list of IP addresses, management of the data becomes more difficult. Fortunately, much of this information can be collected automatically. Several tools that ease the process are described in subsequent chapters, particularly in Chapter 6 .
For process documentation, the best approach is to log and annotate the changes as you make them and then reconstruct the process at a later time. Chapter 11 describes some of the common Unix utilities you can use to automate documentation. You might refer to this chapter if you aren’t familiar with utilities like tee, script, and xwd.
A fundamental assumption of this book is that troubleshooting should be proactive. It is preferable to avoid a problem than have to correct it. Proper management practices can help. While some of this section may, at first glance, seem unrelated to troubleshooting, there are fundamental connections. Management practices will determine what you can do and how you do it. This is true both for avoiding problems and for dealing with problems that can’t be avoided. The remainder of this chapter reviews some of the more important management issues.
To effectively administer a system requires a high degree of professionalism. This includes personal honesty and ethical behavior. You should learn to evaluate yourself in an honest, objective manner. (See The Peter Principle Revisited.) It also requires that you conform to the organization’s mission and culture. Your network serves some higher purpose within your organization. It does not exist strictly for your benefit. You should manage the network with this in mind. This means that everything you do should be done from the perspective of a cost-benefit trade-off. It is too easy to get caught in the trap of doing something “the right way” at a higher cost than the benefits justify. Performance analysis is the key element.
The organization’s mind-set or culture will have a tremendous impact on how you approach problems in general and the use of tools in particular. It will determine which tools you can use, how you can use the tools, and, most important, what you can do with the information you obtain. Within organizations, there is often a battle between openness and secrecy. The secrecy advocate believes that details of the network should be available only on a need-to-know basis, if then. She believes, not without justification, that this enhances security. The openness advocate believes that the details of a system should be open and available. This allows users to adapt and make optimal use of the system and provides a review process, giving users more input into the operation of the network.
Taken to an extreme, the secrecy advocate will suppress information that is needed by the user, making a system or network virtually unusable. Openness, taken to an extreme, will leave a network vulnerable to attack. Most people’s views fall somewhere between these two extremes but often favor one position over the other. I advocate prudent openness. In most situations, it makes no sense to shut down a system because it might be attacked. And it is asinine not to provide users with the information they need to protect themselves. Openness among those responsible for the different systems within an organization is absolutely essential.
We would all like to think that we are irreplaceable, and that no one else could do our jobs as well as we do. This is human nature. Unfortunately, some people take steps to make sure this is true. The most obvious way an administrator may do this is hide what he actually does and how his system works.
This can be done many ways. Failing to document the system is one approach—leaving comments out of code or configuration files is common. The goal of such an administrator is to make sure he is the only one who truly understands the system. He may try to limit others access to a system by restricting accounts or access to passwords. (This can be done to hide other types of unprofessional activities as well. If an administrator occasionally reads other users’ email, he may not want anyone else to have standard accounts on the email server. If he is overspending on equipment to gain experience with new technologies, he will not want any technically literate people knowing what equipment he is buying.)
This behavior is usually well disguised, but it is extremely common. For example, a technician may insist on doing tasks that users could or should be doing. The problem is that this keeps users dependent on the technician when it isn’t necessary. This can seem very helpful or friendly on the surface. But, if you repeatedly ask for details and don’t get them, there may be more to it than meets the eye.
Common justifications are security and privacy. Unless you are in a management position, there is often little you can do other than accept the explanations given. But if you are in a management position, are technically competent, and still hear these excuses from your employees, beware! You have a serious problem.
No one knows everything. Whenever information is suppressed, you lose input from individuals who don’t have the information. If an employee can’t control her ego, she should not be turned loose on your network with the tools described in this book. She will not share what she learns. She will only use it to further entrench herself.
The problem is basically a personnel problem and must be dealt with as such. Individuals in technical areas seem particularly prone to these problems. It may stem from enlarged egos or from insecurity. Many people are drawn to technical areas as a way to seem special. Alternately, an administrator may see information as a source of power or even a weapon. He may feel that if he shares the information, he will lose his leverage. Often individuals may not even recognize the behavior in themselves. It is just the way they have always done things and it is the way that feels right.
If you are a manager, you should deal with this problem immediately. If you can’t correct the problem in short order, you should probably replace the employee. An irreplaceable employee today will be even more irreplaceable tomorrow. Sooner or later, everyone leaves—finds a better job, retires, or runs off to Poughkeepsie with an exotic dancer. In the meantime, such a person only becomes more entrenched making the eventual departure more painful. It will be better to deal with the problem now rather than later.
From the perspective of tools, you must ensure that you use tools in a manner that conforms not just to the policies of your organization, but to all applicable laws as well. The tools I describe in this book can be abused, particularly in the realm of privacy. Before using them, you should make certain that your use is consistent with the policies of your organization and all applicable laws. Do you have the appropriate permission to use the tools? This will depend greatly on your role within the organization. Do not assume that just because you have access to tools that you are authorized to use them. Nor should you assume that any authorization you have is unlimited.
Packet capture software is a prime example. It allows you to examine every packet that travels across a link, including applications data and each and every header. Unless data is encrypted, it can be decoded. This means that passwords can be captured and email can be read. For this reason alone, you should be very circumspect in how you use such tools.
A key consideration is the legality of collecting such information. Unfortunately, there is a constantly changing legal morass with respect to privacy in particular and technology in general. Collecting some data may be legitimate in some circumstances but illegal in others. This depends on factors such as the nature of your operations, what published policies you have, what assurances you have given your users, new and existing laws, and what interpretations the courts give to these laws.
It is impossible for a book like this to provide a definitive answer to the questions such considerations raise. I can, however, offer four pieces of advice:
First, if the information you are collecting can be tied to the activities of an individual, you should consider the information highly confidential and should collect only the information that you really need. Be aware that even seemingly innocent information may be sensitive in some contexts. For example, source/destination address pairs may reveal communications between individuals that they would prefer not be made public.
Second, place your users on notice. Let them know that you collect such information, why it is necessary, and how you use the information. Remember, however, if you give your users assurances as to how the information is used, you are then constrained by those assurances. If your management policies permit, make their prior acceptance of these policies a requirement for using the system.
Third, you must realize that with monitoring comes obligations. In many instances, your legal culpability may be less if you don’t monitor.
Finally, don’t rely on this book or what your colleagues say. Get legal advice from a lawyer who specializes in this area. Beware: many lawyers will not like to admit that they don’t know everything about the law, but many aren’t current with the new laws relating to technology. Also, keep in mind that even if what you are doing is strictly legal and you have appropriate authority, your actions may still not be ethical.
Solutions to problems have economic consequences, so you must understand the economic implications of what you do. Knowing how to balance the cost of the time used to repair a system against the cost of replacing a system is an obvious example. Cost management is a more general issue that has important implications when dealing with failures.
One particularly difficult task for many system administrators is to come to terms with the economics of networking. As long as everything is running smoothly, the next biggest issue to upper management will be how cost effectively you are doing your job. Unless you have unlimited resources, when you overspend in one area, you take resources from another area. One definition of an engineer that I particularly like is that “an engineer is someone who can do for a dime what a fool can do for a dollar.” My best guess is that overspending and buying needlessly complex systems is the single most common engineering mistake made when novice network administrators purchase network equipment.
One problem is that some traditional economic models do not apply in networking. In most engineering projects, incremental costs are less than the initial per-unit cost. For example, if a 10,000-square-foot building costs $1 million, a 15,000-square-foot building will cost somewhat less than $1.5 million. It may make sense to buy additional footage even if you don’t need it right away. This is justified as “buying for the future.”
This kind of reasoning, when applied to computers and networking, leads to waste. Almost no one would go ahead and buy a computer now if they won’t need it until next year. You’ll be able to buy a better computer for less if you wait until you need it. Unfortunately, this same reasoning isn’t applied when buying network equipment. People will often buy higher-bandwidth equipment than they need, arguing that they are preparing for the future, when it would be much more economical to buy only what is needed now and buy again in the future as needed.
Moore’s Law lies at the heart of the matter. Around 1965, Gordon Moore, one of the founders of Intel, made the empirical observation that the density of integrated circuits was doubling about every 12 months, which he later revised to 24 months. Since the cost of manufacturing integrated circuits is relatively flat, this implies that, in two years, a circuit can be built with twice the functionality with no increase in cost. And, because distances are halved, the circuit runs at twice the speed—a fourfold improvement. Since the doubling applies to previous doublings, we have exponential growth.
It is generally estimated that this exponential growth with chips will go on for another 15 to 20 years. In fact, this growth is nothing new. Raymond Kurzweil, in The Age of Spiritual Machines: When Computers Exceed Human Intelligence, collected information on computing speeds and functionality from the beginning of the twentieth century to the present. This covers mechanical, electromechanical (relay), vacuum tube, discrete transistor, and integrated circuit technologies. Kurzweil found that exponential growth has been the norm for the last hundred years. He believes that new technologies will be developed that will extend this rate of growth well beyond the next 20 years. It is certainly true that we have seen even faster growth in disk densities and fiber-optic capacity in recent years, neither of which can be attributed to semiconductor technology.
What does this mean economically? Clearly, if you wait, you can buy more for less. But usually, waiting isn’t an option. The real question is how far into the future should you invest? If the price is coming down, should you repeatedly buy for the short term or should you “invest” in the long term?
The general answer is easy to see if we look at a few numbers. Suppose that $100,000 will provide you with network equipment that will meet your anticipated bandwidth needs for the next four years. A simpleminded application of Moore’s Law would say that you could wait and buy similar equipment for $25,000 in two years. Of course, such a system would have a useful life of only two additional years, not the original four. So, how much would it cost to buy just enough equipment to make it through the next two years? Following the same reasoning, about $25,000. If your growth is tracking the growth of technology, then two years ago it would have cost $100,000 to buy four years’ worth of technology. That will have fallen to about $25,000 today. Your choice: $100,000 now or $25,000 now and $25,000 in two years. This is something of a no-brainer. It is summarized in the first two lines of Table 1-1.
Table 1-1. Cost estimates
Four-year plan with maintenance
Two-year plan with maintenance
Four-year plan with maintenance and 20% MARR
Two-year plan with maintenance and 20% MARR
If this argument isn’t compelling enough, there is the issue of maintenance. As a general rule of thumb, service contracts on equipment cost about 1% of the purchase price per month. For $100,000, that is $12,000 a year. For $25,000, this is $3,000 per year. Moore’s Law doesn’t apply to maintenance for several reasons:
A major part of maintenance is labor costs and these, if anything, will go up.
The replacement parts will be based on older technology and older (and higher) prices.
The mechanical parts of older systems, e.g., fans, connectors, and so on, are all more likely to fail.
There is more money to be made selling new equipment so there is no incentive to lower maintenance prices.
Thus, the $12,000 a year for maintenance on a $100,000 system will cost $12,000 a year for all four years. The third and fourth lines of Table 1-1 summarize these numbers.
Yet another consideration is the time value of money. If you don’t need the $25,000 until two years from now, you can invest a smaller amount now and expect to have enough to cover the costs later. So the $25,000 needed in two years is really somewhat less in terms of today’s dollars. How much less depends on the rate of return you can expect on investments. For most organizations, this number is called the minimal acceptable rate of return (MARR). The last two lines of Table 1-1 use a MARR of 20%. This may seem high, but it is not an unusual number. As you can see, buying for the future is more than two and a half times as expensive as going for the quick fix.
Of course, all this is a gross simplification. There are a number of other important considerations even if you believe these numbers. First and foremost, Moore’s Law doesn’t always apply. The most important exception is infrastructure. It is not going to get any cheaper to pull cable. You should take the time to do infrastructure well; that’s where you really should invest in the future.
Most of the other considerations seem to favor short-term investing. First, with short-term purchasing, you are less likely to invest in dead-end technology since you are buying later in the life cycle and will have a clearer picture of where the industry is going. For example, think about the difference two years might have made in choosing between Fast Ethernet and ATM for some organizations. For the same reason, the cost of training should be lower. You will be dealing with more familiar technology, and there will be more resources available. You will have to purchase and install equipment more often, but the equipment you replace can be reused in your network’s periphery, providing additional savings.
On the downside, the equipment you buy won’t have a lot of excess capacity or a very long, useful lifetime. It can be very disconcerting to nontechnical management when you keep replacing equipment. And, if you experience sudden unexpected growth, this is exactly what you will need to do. Take the time to educate upper management. If frequent changes to your equipment are particularly disruptive or if you have funding now, you may need to consider long-term purchases even if they are more expensive. Finally, don’t take the two-year time frame presented here too literally. You’ll discover the appropriate time frame for your network only with experience.
Other problems come when comparing plans. You must consider the total economic picture. Don’t look just at the initial costs, but consider ongoing costs such as maintenance and the cost of periodic replacement. As an example, consider the following plans. Plan A has an estimated initial cost of $400,000, all for equipment. Plan B requires $150,000 for equipment and $450,000 for infrastructure upgrades. If you consider only initial costs, Plan A seems to be $200,000 cheaper. But equipment needs to be maintained and, periodically, replaced. At 1% per month, the equipment for Plan A would cost $48,000 a year to maintain, compared to $18,000 per year with Plan B. If you replace equipment a couple of times in the next decade, that will be an additional $800,000 for Plan A but only $300,000 for Plan B. As this quick, back-of-the-envelope calculation shows, the 10-year cost for Plan A was $1.68 million, while only $1.08 million for Plan B. What appeared to be $200,000 cheaper was really $600,000 more expensive. Of course, this was a very crude example, but it should convey the idea.
You shouldn’t take this example too literally either. Every situation is different. In particular, you may not be comfortable deciding what is adequate surplus capacity in your network. In general, however, you are probably much better off thinking in terms of scalability than raw capacity. If you want to hedge your bets, you can make sure that high-speed interfaces are available for the router you are considering without actually buying those high-speed interfaces until needed.
How does this relate to troubleshooting? First, don’t buy overly complex systems you don’t really need. They will be much harder to maintain, as you can expect the complexity of troubleshooting to grow with the complexity of the systems you buy. Second, don’t spend all your money on the system and forget ongoing maintenance costs. If you don’t anticipate operational costs, you may not have the funds you need.
 Do you know what operating system I’m tactfully not naming?
 Admittedly these guidelines are ideals. Does anyone actually do all of this documenting? Yes, while most administrators probably don’t, some do. But just because many administrators don’t succeed in meeting the ideal doesn’t diminish the importance of trying.
 This is a pretty big if, but it’s reasonable for most users and organizations. Most users and organizations have selected a point in the scheme of things that seems right for them—usually the latest technology they can reasonably afford. This is why that new computer you buy always seems to cost $2500. You are buying the latest in technology, and you are trying to reach about the same distance into the future.