DCD...DSR....!....DTR......?.....RTS......@...CTS....!

In ethernet : Ethernet0 is down, Line Protocol is down => interface problem

In Serial link: Serial0 is down, Line Protocol is down

=> When on a Serial interface something else might be the problem. Serial interfaces operate in pairs and there usually is a CSU/DSU operating between them along the media. So you need to investigate the signals at the bottom of a show interface

DCD = Data Carrier Detect
DSR = Data Set Ready
DTR = Data Terminal Ready
RTS = Request to Send
CTS = Clear to Send
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When you connect two routers via a crossover cable one router will be the DTE (Data Terminal Equipment) and the other router will be the DCE ( Data Circuit-Terminating Equipment). You need to configure the clocking on the DCE. This is accomplished by entering this command router#(config-if)clockrate.

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When the routers are connected via a WAN link both routers are the DTE. With CSU/DSU's acting as the DCE providing the clocking.

The DTE is responsible for DTR and RTS. The DCE is responsible for DCD, DSR and CTS.

i.e. DTE=> DTR, RTS
DCE=>DCD, DSR and CTS
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Scenarios:
DCE router.
Serial0 is down, Line Protocol is down
DCD=up DSR=up DTR=down RTS=down CTS=up
This indicates that the DTE router is down or a media problem.

DTE router connected via a WAN link.
Serial0 is down, Line Protocol is down
DCD=down DSR=down DTR=up RTS=up CTS=down
This indicates a problem with the media or the CSU/DSU.

Two routers connected via crossover cable.
Serial0 is down, Line Protocol is down
DCD=down DSR=down DTR=up RTS=up CTS=down
This indicates that the DCE router is down or a media problem.
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Serial0 is up, Line Protocol is down
This shows that Serial0 is receiving physical layer clocking. But you may have a problem with the media or there is an encapsulation mismatch.

Serial0 is up, Line Protocol is up
This shows that Serial0 is receiving physical layer clocking and that the router is getting hello/keepalives from the remote end.
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