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  1. Junior Member
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    #1

    Default FastEthernet0 is up, line protocol is down..Why?

    Hope someone can help or explain to me this situation. To me this would be normal:

    FastEthernet0 is up, line protocol is up: Both the Physical and Data Link layers on the interface are functioning correctly.
    FastEthernet0 is down, line protocol is down: This output indicates a physical interface problem. For example, the cable on this interface or on the remote interface is disconnected.
    FastEthernet0 is up, line protocol is down: In this case, Physical layer is operational. The line protocol being down indicates a clocking or framing problem. Probable reasons for this are encapsulation and clock rate mismatches.
    Ethernet0 is administratively down, line protocol is down: This output indicates that a local interface has been manually shut down using the shutdown command.
    ----------------

    I have Cisco 1841 router and this is what is happening:
    I configure the interface fa0/0 with IP address and no shut(the cable is not attached to this interface).
    The interfece goes to up and up state...and after a while to protocol down state.

    When I do show ip int brief I get this message:
    FastEthernet0 is up, line protocol is down

    I think that the message should be:
    FastEthernet0 is down, line protocol is down
    (no cable is attached to interface)

    Will the interface go to down state, why is this happening?

    Tnx
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  3. The Undeniable!!! jahsoul's Avatar
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    #2
    If possible could you throw the configs on here and what it is connected to. Do they share the same basic configs? (i.e. speed and duplex)

    But from my limited knowledge-

    1)Double check the cabling. I've had the problem were I assumed that it couldn't be a physical issue but it was.

    2)Check speed and duplex settings
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  4. Junior Member
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    #3
    @jahsoul
    Tnx for replay!
    The thing is that there is no config ont the router.. It's my lab router..it has no startup-config..nothing at all..
    Only thing that I do is:
    conf t
    int fa 0/0
    no shut

    the router shows me this massage: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

    and after when i do ''show ip int brief'':
    Interface IP-Address OK? Method Status Protocol
    FastEthernet0/0 unassigned YES unset up down


    and the Status stays in ''up'' state - it won't go to down state
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  5. M&C: Far Side of the Net vinbuck's Avatar
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    #4
    If I remember correctly, you can actually have an interface show as up even if all of the pairs aren't terminated properly in the Cat5/6 cable.

    Check

    1) Cabling - use a cable that is known to be working
    2) Duplex - either Auto/Auto on both sides or hard coded for speed and duplex
    3) IOS - are you using a stable IOS?
    4) Change the device it is connected to on the other end to see if there is any change
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  6. Senior Member advanex1's Avatar
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    #5
    Agreed with what the others said. Check the cabling with a known good cable. Also, I'd double check to make sure you have correct encapsulation or clock rate as well.
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  7. Junior Member
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    #6
    As I said in my first post
    I configure the interface fa0/0 with IP address and no shut(the cable is not attached to this interface).

    So ther is no cable attached to the interface...I'm connected using console cable..

    I have one brand new router..I will try that one..

    here is something that I found

    I don't think it's just Fast Ethernet. I've seen "Slow" Ethernet report
    > > that the interface is up, down when nothing was attached. It seems to
    just
    > > be an indication that the interface passed a hardware check.


    Re: FastEthernet is up, Line protocol down [7:1828]
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  8. Senior Member
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    #7
    There is nothing wrong with your router.

    If you issue the No Shut command on your fast ethernet interface even when no cable is attached it will show as: Up Down

    Most books will tell you it should say: Down Down, but as you've just found out in real life (not a simulator) it doesn't. However if you repeat the same No shut command on a serial interface it with no cable attached it will show as Down Down.
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  9. Junior Member
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    #8
    The same thing happens with the brand new router...out of the box..

    Thanks for the replays..
    Last edited by janez_drk; 07-11-2011 at 02:03 PM.
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  10. Senior Member
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    #9
    Sorry to bring this back up but why does it show up/down with ethernet but down/down with serial?

    How would you troubleshoot a bad ethernet cable if it shows up when no cable is even connected?
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  11. Network Consultant FloOz's Avatar
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    #10
    If the other end of the cat cable is not attached to an interface you can do a no keepalive cmd on fa0/0 and the interface will then show as up/up
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  12. Senior Member
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    #11
    I understand that, but that still doesn't explain the difference between serial and ethernet down/down and up/down status difference or how to know if an ethernet cable is faulty.

    Does serial work differently requiring a keepalive for the other device but ethernet doesnt?
    Last edited by wallpaper_01; 05-09-2013 at 11:33 AM.
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