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.