Comes up at 100 Mb, not 1 Gb

Likely causes

• Open or high-resistance conductor on 4,5 or 7,8

  ~35%

  100BASE-TX only uses 1,2 and 3,6, so a fault confined to the other two pairs is invisible to the user and to the link light. A wiremap names it in seconds, and pinning the port to 1000 drops the link.

• Run reused from voice: only two pairs ever landed

  ~25%

  4,5 and 7,8 dead at the jack, and the far end is on a 66 or 110 field, a splitter, or still feeding a phone. Measured length often doesn't match the drawing either.

• Split or reversal confined to 4,5 and 7,8

  ~15%

  Continuity passes, the certifier calls the split or reversal on those pairs, and NEXT fails on combinations involving them. 100 Mb is rock solid because those pairs are unused.

• Port, NIC, driver, or speed and duplex forced at one end

  ~12%

  The fault follows the port or the NIC when you move the same cord elsewhere, or one end's interface config is pinned to 100. Free to rule out, so do it first.

• Damaged conductor mid-run on one of the two unused pairs

  ~10%

  A distance on the failing pair only, with the other three clean. Staple, screw, tray edge, firestop sleeve.

• Two-pair cord or two-pair keystone in the path

  ~3%

  Swapping the cord fixes it instantly. Count the conductors in the plug: four conductors cannot do gigabit.

What to bring

• Certifier, or at minimum a wiremap tester that reads all four pairs
• Switch CLI or NMS access for speed and counters
• Tone generator and probe
• Spare certified 8-conductor cords
• Punch tool with correct blade
• Jack instruction sheet (phone)
• Inspection light
• Ladder or lift

Steps

1. 1

   Step 1: Read the interface at both ends before you climb

   Speed, duplex, auto-negotiation state, error counters. Then move the device to a known-good port. A forced setting or a dead port costs nothing to rule out and it is the answer often enough to check first.

   If that doesn’t do it

   Both ends auto-negotiating and the port proves good elsewhere: the cabling owns it.

2. 2

   Step 2: Swap the patch cords and count the conductors

   Two-pair cords and two-pair keystones still turn up on reused voice runs. Look into the plug: eight conductors, or it will never do gigabit.

   If that doesn’t do it

   Cords are 8-conductor and category rated: test the link.

3. 3

   Step 3: Wiremap it

   This is the whole symptom most of the time. Any certifier, and most decent wiremap testers, will name an open, short, reversal or split on 4,5 and 7,8 in seconds. Check all four pairs, not the two that are working.

   If that doesn’t do it

   Wiremap clean on all four pairs: certify it properly and read insertion loss, NEXT and DC resistance.

4. 4

   Step 4: Follow the distance on the failing pair

   Pace it out and inspect. Near 0 m is the end you're plugged into, at link length is the far termination, in between is damage at that point.

   If that doesn’t do it

   Nothing at the distance: look at the far end's history.

5. 5

   Step 5: Go and look at the far end properly

   Old 110 or 66 block, a splitter, a phone still landed on 4,5, or a pair borrowed for something else. Reused runs carry decades of other people's decisions.

   If that doesn’t do it

   Far end is a clean eight-conductor termination: re-terminate both ends.

6. 6

   Step 6: Re-terminate or re-pull

   Unplug the switch port first. Re-punch to the jack sheet with all four pairs landed. One damaged conductor is not repairable to category. Re-pull the run.

   If that doesn’t do it

   New run still negotiates 100: you have the wrong run identified. Tone it out and re-label.

7. 7

   Step 7: Prove the fix at the port, then certify

   Confirm 1000 full duplex and clean error counters at both ends over a shift, then certify the link and file the result. 1000BASE-T needs auto-negotiation and all four pairs, so a clean negotiation is the proof.

   If that doesn’t do it

   Negotiates 1000 but throws errors: go to Intermittent link that certifies clean, or Cat6A 10G errors after a clean cert if this is a 10G port.

More Copper & Certification guides

• Wiremap fail, crossed or reversed pairs
• NEXT failing at the panel end
• Split pair, continuity fine, NEXT gone
• Whole job failing, or passing too well
• PoE device won't power, or reboots under load

All 54 guides