If I have to troubleshoot a network stating that one of the interface is not working (i.e, a link is down), what steps would I need to take to make it working again?


Handy way to start troubleshooting is from Physical Layer following OSI Layer Model

  1. Check physical port/cable (as mentioned by @Zac67 also)
  2. Check port negotiation & Duplex, speed on both ends.
  3. If ports status are same & verified, move to cable part (Fiber, Ethernet CAT5/6 what ever you have ..) if you have Optical Patch cord.. Try looping one end of patch cord by using looper (if you connect one end with looper, port on other end will be showing UP confirming fiber patch is OK

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    Twisted-pair ports can be tested with a loop adapter as well. – Zac67 Apr 5 '20 at 12:38
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    Well, I would say you are right and go from layer 1 to 7 and check everything like that. – Bulki Apr 5 '20 at 14:28

If the link is physically down, the steps are very easy.

  1. Check port configurations (shutdown, manual speed/duplex selection).
  2. Check port compatibility (twisted-pair ports are not all downwards compatible, e.g. 10GBASE-T and 10BASE-T; fiber PHYs).
  3. Check the cable and connectors (correct type/grade/length, visible damage, kinks, crushing, overstretching, damage or dirt on connectors, correct seating).

Most of these can very easily be tested with a known-good device/port/cable or a loopback adapter, if available.

Examining the device logs for anomalies may also be helpful.

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    4. Check using a known working port. – YLearn Apr 5 '20 at 14:08
  • @YLearn Thx, thought that was obvious - it all is, actually... – Zac67 Apr 5 '20 at 14:33

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