Layer 1 errors can include:

• Broken cables
• Disconnected cables
• Cables connected to the wrong ports
• Intermittent cable connection
• Wrong cables used for the task at hand (must use rollovers, crossover cables, and straight-through cables correctly)
• Transceiver problems
• DCE cable problems
• DTE cable problems
• Devices turned off

Layer 2 errors can include:

• Improperly configured serial interfaces
• Improperly configured Ethernet interfaces
• Improper encapsulation set (HDLC is default for serial interfaces)
• Improper clockrate settings on serial interfaces
• Network interface card (NIC) problems

Layer 3 errors can include:

• 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.

Interactive Media Activity Matching: Testing with the OSI layers After completing this activity, the student will be able to understand the OSI layers.

Web Links General Troubleshooting Tools http://www.nwnetsmart.com/ns/books/ ciscopress/samples/ 1578700922.pdf