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| 3.1 |  | Characteristics of Physical Layer Failure Problems |
| 3.1.5 |  | Noncritical characteristics – console messages |

With a physical layer failure problem, sometimes a problem is discovered when a device shows console messages indicating that an interface is not functioning. For instance, the show interfaces command for a failing interface may show one of the following:

- interface is down, line protocol is down (FastEthernet)
- initialized, down state (Token Ring)

An unattached Ethernet LAN interface can be spoofed to an interface is up, line protocol is up state by issuing the no keepalive command. Therefore, this command should not be used in a live network as a substitute for testing at the physical layer.

Figure shows possible problems that can cause the line status or the line protocol to be down.

In a Frame Relay environment, troubleshooting is not so simple. For example, if the circuit goes down at one side of the non-broadcast network, but the connection to the local loop on the other side is functional and configured on a physical interface, then the interface for that device will still show as administratively up and line protocol up. The connection of the remote router to the local switch is still up, even though the circuit with its partner is down.

If the connection is configured with subinterfaces on both ends, then a loss of connectivity on either side will cause both subinterfaces to go down.