OUTPUT 101- Interface states

Sometimes it is necessary to go back to the basics that we have already forgotten. You can identify six possible states in the interface status line of the show interfaces serial output:

  1. Serial x/y is up, line protocol is up
  2. Serial x/y is down, line protocol is down
  3. Serial x/y is up, line protocol is down
  4. Serial x/y is up, line protocol is up (looped)
  5. Serial x/y is up, line protocol is down (disabled)
  6. Serial x/y is administratively down, line protocol is down

1- Serial x/y is up, line protocol is up

  • This is what you want to see, the proper line status condition, you should have end-to-end connectivity.

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2- Serial x is down, line protocol is down

  • Remote end interface admin shutdown. (specifically p-2-p)
  • Cable Unplugged or faulty at local or remote end.
  • Hardware failure has occurred (CSU/DSU).

Possible Solutions:

  • Start by confirming the other interface is not shutdown
  • Try replacing the cabling.
  • Replace the various hardware components.

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3- Serial x is up, line protocol is down

  • Could be local or remote misconfiguration, example, encapsulation mismatch.
  • Keepalives are not being sent by the remote router.
  • A local or remote CSU/DSU has failed.
  • End-to-End fault within Carrier Provider network. (either noisy link, failed switch, or misconfiguration)

Possible Solutions:

  • Confirm encapsulations are matching and interface specifc configurations are correct.
  • Try putting the line in local loopback mode and use see if the line protocol comes up, with “sh interface”. If the line protocol comes up, a failed remote device is the likely problem.  If the line protocol does not come up in local loopback mode, router hardware could likely be the problem. (This solution will only work with HDLC encapsulation)
  • For FR and PPP encapsulation a looped interface will always have the line protocol down, so protocol specific Debug commands could be used to see if the keepalive increases. (ALWAYS limit your  debug to only the necessary interface)
    • PER-INTERFACE : debug serial interface x/y
    • HDLC: debug serial interface
    • PPP: debug ppp negotiation
    • FR: debug frame-relay lmi
  • If the line protocol comes up and the keepalive counter increments, the problem is not the local router.
  • Verify all cabling. Make certain that the cable is attached to the correct interface.
  • Replace the various hardware components.

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4- Serial x is up, line protocol is up (looped)

  • A loop exist on the link. Either local or in Carrier Network.
  • This is dedected when a already sent random keep-alives number is returned apose to increased  keep-alive number.

Possible Solutions:

  • Check the interface configuration for the “loopback” command. If exist, remove.
  • Contact your Carrier Provider.

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5- Serial x is up, line protocol is down (disabled)

  • Although an uncommon state, this generally points to hardware failure.

Possible Solutions:

  • Examine the output of “show controller serial x/y”
  • Swap out bad hardware, as required (CSU/DSU, switch, local or remote interface cards or router).

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6- Serial x is administratively down, line protocol is down

  • The “shutdown” command was added to the interface by an admin.
  • Duplicate interface IP existed on the local router at the time of reload.

Possible Solutions:

  • Use the “no shutdown” interface command.
  • Remove the duplicate IP of the errornous interface.
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