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Troubleshooting Layer 2 using show interfaces

Troubleshooting Layer 2 using show interfaces 
9.3.2 This page will further explain why the show interfaces command may be the most important tool to discover Layer 1 and Layer 2 problems with the router. The first parameter, which is line, refers to the physical layer. The second parameter, which is protocol, indicates if the IOS processes that control the line protocol consider the interface usable. This is determined by whether keepalives are successfully received. Keepalives are defined as messages sent by one network device to inform another network device that the virtual circuit between the two is still active. If the interface misses three consecutive keepalives, the line protocol is marked as down.
When the line is down, the protocol is always down, because there is no useable media for the Layer 2 protocol. This will be true when the interface is down due to a hardware problem and when it is administratively down.
If the interface is up and the line protocol is down, a Layer 2 problem exists. Possible causes are as follows:
  • No keepalives
  • No clock rate
  • Mismatch in encapsulation type
The show interfaces command should be used after configuring a serial interface to verify the changes and that the interface is operational.
The Interactive Media activity will help students understand the show interfaces command.
The next page will discuss the show cdp command

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