Cisco – What can cause “FastEthernet is up, line protocol is down” on a Cisco Switch

ciscoethernetinterfaceswitch

I wonder what can cause the first line output of the command show interfaces to be: FastEthernet is up, line protocol is down.

In other words,

  • Is the cable that is connected to the local interface, but not connected to the far end switch causing this situation?
  • Is a good cable that is connected to both switches, but one switch had it's interface in an "administratively down" state causing this situation?

If not, what would cause an interface to be in an up,down state?

Best Answer

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
sw1#

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#
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
sw1#

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.

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