10

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?

Kevin Bowen
  • 260
  • 1
  • 4
  • 12
kolxxx
  • 101
  • 1
  • 1
  • 4
  • Though I am new to networking, I was testing a simulation on a network through Cisco packet tracer and one serial interface had up/ down. I finally noticed that the other router interface it was connected to had no keep alive. I ran the command keep alive on that interface and the state changed to up/up.. – ITNewbie Mar 01 '16 at 05:43
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer. – Ron Maupin Jan 03 '21 at 05:10

4 Answers4

12

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.

Mike Pennington
  • 29,989
  • 11
  • 79
  • 152
0

sometimes is the encapsulation mismatch. whick menas that keepalives will not be receiver on the far end.

So it's a datalink layer problem.

claudio
  • 1
0

In my case, use different prefix length can cause this problem.

For example, on switch interface configure an IP address 192.0.2.0/31 and on server interface configure 192.0.2.1/24.

qin
  • 123
  • 8
  • IP addressing, at layer-3, does not affect the layer-1/2. – Ron Maupin Sep 25 '18 at 23:39
  • @RonMaupin But, when I config ip address with /31 mask, it become a real p2p link. So I think it may affect layer 2. – qin Sep 26 '18 at 02:50
  • No, it is point-to-point at layer-3. Switch interfaces are layer-2 interfaces, but if you can address it, then you have enabled layer-3 and it is a router interface. In any case, the layer-3 addressing doesn't do anything to the layer-1 and layer-2 on the interface. In the question, the interface is layer-1, and the line protocol is layer-2. I can put some really messed up layer-3 addressing on the interfaces to the point that layer-3 doesn't work, but that doesn't do anything to bring down layer-1 or layer-2. – Ron Maupin Sep 26 '18 at 02:55
  • @RonMaupin When I change server ip from 192.0.2.1/24 to 192.0.2.1/31, the line protocol is change to UP – qin Sep 27 '18 at 06:12
-1

it is a speed and / or duplex mismatch

user19666
  • 1
  • 4
    A better answer would include the reasons. A statement without backup is typically suspect. You should explain/justify your conclusion. – Ron Maupin Oct 12 '15 at 04:00