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Testing by OSI layers

Testing by OSI layers
9.2.3 This page will describe the types of errors that occur at the first three layers of the OSI model.
Layer 1 errors can include the following: 
  • Broken cables
  • Disconnected cables
  • Cables connected to the wrong ports
  • Intermittent cable connection
  • Rollover, crossover, or straight-through cables used incorrectly
  • Transceiver problems
  • DCE cable problems
  • DTE cable problems
  • Devices turned off
Layer 2 errors can include the following: 
  • Improperly configured serial interfaces
  • Improperly configured Ethernet interfaces
  • Improper encapsulation set
  • Improper clockrate settings on serial interfaces
  • Network interface card (NIC) problems
Layer 3 errors can include the following: 
  • Routing protocol not enabled
  • Wrong routing protocol enabled
  • Incorrect IP addresses
  • Incorrect subnet masks
If errors appear on the network, the process of testing through the OSI layers should begin. The ping command is used at Layer 3 to test connectivity. At Layer 7 the telnet command may be used to verify the application layer software between source and destination stations. Both of these commands will be discussed in detail in a later section.
The next page will explain how indicator lights can be used to test a network.

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