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Sams Teach Yourself Network Troubleshooting in 24 Hours, Second Edition by Jonathan Feldman

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Using the “Important Parts”

You’ll never arrive at a final solution by using OSI, but you’ll use it as an important targeting mechanism. When you’re in the targeting phase of a problem (that is, when you’re first figuring out where to focus your efforts), you should start to run through the OSI layers that matter (1, 2, 3, and 7), but instead of running through them in order (from 1 to 7), go from top to bottom (7 to 1).

Because each layer relies on the next, it makes sense to ask, “Does the summation of everything work?” (the Application layer) before moving on to the next thing. See Figure 2.2 for a flowchart of this. Should you always ask about layer 7 first? Well, no. There are certainly times when you should jump right to layer 1, like ...

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