Difference between X is up and line protocol is up?

I'm new to the Cisco stuff and I've been playing around with configuring the interfaces of routers and just want to clarify about what does X is up/down or administratively down, and line protocol is up/down status' mean and all the different combinations.  What exactly does line protocol mean?  Which status relates to the actual cable being in/out and which relates to the 'no shutdown'/'shutdown' commands?

Thanks
kdavidmAsked:
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Don JohnstonInstructorCommented:
One (simple) way to think about it is this:

If the interface is down, the cable isn't connected, or the device that the cable connects to is not powered.
If the line protocol is down, well that one's a little more complicated but basically it means that information is unable to move on the wire.
If you've issued a "shutdown" on the interface then it will show as being "adminsitrately shutdown".

From Cisco:

Interface: (Ethernet)
Indicates whether the interface hardware is currently active and if it has been taken down by an administrator. "Disabled" indicates the router has received over 5000 errors in a keepalive interval, which is 10 seconds by default.
 
line protocol: (Ethernet)
Indicates whether the software processes that handle the line protocol believe the interface is usable (that is, whether keepalives are successful) or if it has been
 
Interface: (Serial)
Indicates whether the interface hardware is currently active (whether carrier detect is present) or if it has been taken down by an administrator.
 
line protocol: (Serial)
Indicates whether the software processes that handle the line protocol consider the line usable (that is, whether keepalives are successful) or if it has been taken down by an administrator.
 
http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113ed_cr/fun_r/frprt3/frshowse.htm
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pjtemplinCommented:
Interface is up/down refers to layer 1.
Line protocol is up/down refers to layer 2.

Obviously, layer 1 needs to come up for layer 2 to come up.  

For a serial interface to come up, the circuit normally needs to be good to the far-side CSU.

Ports in 'shutdown' are administratively down.
Other possibilities are:
Interface down, line protocol down.
Interface up, line protocol down.
Interface up, line protocol up.

It's impossible for the interface down with line protocol up.
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