Troubleshooting Circuits

We as engineers spend a lot of time troubleshooting circuits that are down. These are my troubleshooting steps and I use the checklist for when the carrier says it’s a customer issue and not their equipment.

I like to start at physical layer and work my way up the OSI model.

  • Check the smartjack. Is it all green? If not check the amber leds and notify the carrier which ones are amber.
  • Check the cable between the smartjack and the router. If the carrier extended the circuit this is their responsibility, but since we want the circuit up ASAP I test it.
  • Loopback the card (T1/T3/whatever). This will tell us if the card is good. We should have all green led’s on the card.
  • Swap the WAN card if possible
  • Put the WAN card in another slot (this has never resolved an issue for me, but I do it anyway)


  • Check the logs (probably already have)
  • Check the interface statistics. Look for CRC errors and input/output drops
  • Check the clocking on the line
  • Double check the lincode and framing
  • Check for increasing errors on the controller
  • Reboot the router. I hate doing this but I have seen where a carrier will move the T1 to a new card on their side and a reboot of my router is needed.
I’ll try and flush some of this info out as I run into outages. Hopefully it takes a while :-)

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