command is perhaps the single most important tool to discover Layer 1 and Layer 2 problems with the router. The first parameter (line) refers to the physical layer. The second parameter (protocol) indicates whether 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. Among the possible causes are:

• No keepalives
• No clock rate
• Mismatch in encapsulation type

The show interfaces serial command should be used after configuring a serial interface to verify the changes and that the interface is operational.

Interactive Media Activity Drag and Drop: show interface interface command After completing this activity, the student will be able to understand show interface interface command.

Web Links show ip interface EXEC Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_command_reference_ chapter09186a0080087387.html#1020817