To that end, network-management products use the Oreo approach, starting in the middle and working their way out. The creamy Layer 3 gets mapped with a ping sweep, then the tool moves outward, trying SNMP, TCP and UDP ports. The good tools will attempt to piece together a Layer 2 topology, mapping the MAC (Media Access Control) addresses attached to specific switch ports. Of course, in a shared-hub network, Layer 2 goes out the window, but even in all-switched networks, correctly identifying devices attached to specific ports isn't always possible. This is partly because a specific MAC address will most likely show up in a multiple-switches cache. If the device is supporting the Bridge MIB, it's possible for the software to figure out what's attached. Unfortunately, not all SNMP is created equal, and sometimes the information is reported incorrectly. I've yet to find network-management software, complex and expensive or cheap and simple, that gets these Layer 2 mappings right 100 percent of the time.
Additionally, no network tool can map the total physical layer "automagically." Most cable testers will map pin outs, cable length, physical path attenuation and cross talk. But they can't tell you what patch panel is used and what conduit the path takes--there's no replacement for those old-fashioned manual drawing tools.
As with all products, you get what you pay for, but this doesn't mean free products are risky. Most free network-management tools perform tasks so simple and so straightforward--pinging, looking up names, serving files--you can be confident they'll work. For this simple stuff, support just isn't that important.
But as you rely on deeper functionality, such as packet capture, decode and network-performance assessment, you'll need to look more closely at support. MRTG (Multi Router Traffic Grapher), which is free and graphs SNMP values, has a great support community. However, everyone within that community contributes only out of his or her good graces. So if you don't have the time, you'd better have the money to invest in a fully supported corporate (read: expensive) solution.
Cheap network utilities and network-management products are easy to find and easy to use. So why would anyone consider the big, expensive network-management suites from the likes of Computer Associates, IBM and BMC Software? It depends on your organization's goals. If the network is making money or directly supports money-making transactions, you probably have to bite the bullet and invest in a centralized relational database with common data formats and maintenance contracts--it will be worth the cost.
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