I wonder what can cause that the first line output of the command "show interfaces" will be: "fastEthernet is up, line protocol is down".
Cisco ethernet interfaces are normally down / down if they don't have a link. If you're seeing up / down, the most likely causes are:
- Cable fault
- Speed mismatch (I personally haven't seen a duplex mismatch bring an intf up / down)
- is cable that connected to the local interface, but not connected to the far end switch, will cause that situation?
If the cable is bad...
- is good cable that connected to both switches, but one switch had it's interface in "administratively down" state, will cause that situation?
I haven't seen that recently. For example, I have a c3560c in my lab and shutdown fa0/12... then I connected a good cable between the fa0/11 and fa0/12 ports...
sw1#sh ip int brief | i 0/1[1-9]
FastEthernet0/11 unassigned YES unset down down
FastEthernet0/12 unassigned YES unset administratively down down
That said, I do have vague memories of seeing up / down when the remote interface was shut on other platforms in the past, but I don't remember seeing it recently
If the cable is faulty, it could cause up / down status
Testing your cabling:
If you have a Cisco switch, you can test your cabling on the up / down interface like this... the following is good
tdr output for the command when nothing is connected to the other end of the cable.
sw1#test cable-diagnostic tdr interface Fa0/6
TDR test started on interface Fa0/6
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
sw1#show cable-diagnostics tdr interface fa0/6
TDR test last run on: February 12 04:45:37
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/6 auto Pair A 31 +/- 1 meters N/A Open
Pair B 31 +/- 1 meters N/A Open
Pair C N/A N/A Not Supported
Pair D N/A N/A Not Supported
Note: FastEthernet interfaces by-definition can only test two of the four pairs. GigabitEthernet interfaces can test all four pairs.
Older switches don't have a tdr function... you'd have to test the cabling manually.