Fiber TX-RX Polarity Check: Both Ends Show Light but the Link Stays Down

This is the fault that eats an hour and produces a perfectly clean meter reading. Light levels are fine at both ends, the optics pass, the patch cords are known-good — and the link stays down. The usual suspect is not a part at all: it is polarity. Somewhere between the two switches, the pair got crossed, so each transmitter is shouting into the other transmitter and neither receiver hears anything. The check takes five minutes once you stop swapping parts and start testing the pair.

The rule: TX must land on RX

A duplex fiber link is two one-way streets. Side A transmits on fiber 1 and receives on fiber 2; side B must receive on fiber 1 and transmit on fiber 2. Every patch panel, cross-connect, and adapter in between has to preserve that flip exactly once across the whole path. Cross it zero times or twice and the link works; cross it once or three times and it does not — even though every individual segment tests fine on its own.

That middle row is the trap: a meter cannot tell you polarity. It only measures power, and a crossed pair delivers full power to the wrong receiver. If your readings are healthy and the link is still down, stop measuring light and start checking the pair.

The check, in order

1. Loopback-test each optic. Connect each optic's TX to its own RX with a known-good loopback cord (or a short known-good patch cord looped at the panel). If the port comes up, the optic and the port are good. Do both ends. This rules out the only two parts a swap would have "fixed" anyway.
2. Verify the pair map, not the labels. Read the actual fiber positions at each panel and adapter: which position carries side A's TX, which carries side B's TX. Labels lie after a re-patch; positions do not. A common floor cause is a pair re-terminated one position over after a move, or a duplex clip flipped during cleaning.
3. Swap the pair at ONE end only. Flip the duplex pair at one panel — one end, one change — and re-test. If the link comes up, the pair was crossed and you just fixed it. If you swap at both ends, you have changed the polarity twice and proven nothing; the link state tells you nothing about where the cross was.
4. Trace the cross to its panel. Once the link is up, walk the panels and find where the flip actually happened: the adapter, cassette, or cross-connect where TX met TX. Fix it there properly (re-terminate or re-patch in the correct order) rather than leaving a compensating swap somewhere else — a compensating swap becomes the next tech's mystery cross.
5. Re-verify and document. Confirm the link is up with zero errors, then write the ticket line below. If the link is still down after the swap, polarity is ruled out: go back to the loss fault path — inspect, clean, and re-measure.

MPO and trunk polarity: the three methods

On MPO trunk links the same rule applies at cassette level, and the method matters. Method A keeps the same fiber positions end to end (straight), Method B flips them (key-up to key-down), Method C flips pairs. Mixing methods across a trunk — say a Method A cassette on a Method B trunk — produces exactly the crossed-pair symptom above, at 12 fibers at once. When a whole MPO link is dark with good light, check that every cassette and trunk in the path uses the same method before touching any optic.

Recording a polarity fix in the ticket

A polarity fix that is not written down will be "fixed" again by the next person who re-patches the panel. The minimum useful line:

SW-04 xe-0/0/5 ↔ SW-07 xe-0/0/5 via ODF-A 14 / ODF-B 22.
Loopback pass both ends (optics + ports good). Light levels
normal both directions, link down → polarity suspected.
Swapped duplex pair at ODF-B only → link up, 0 errors.
Root cause: pair crossed at ODF-B cassette (TX→TX).
Corrected at ODF-B; do NOT re-swap at ODF-A.

The shape to copy: port pair, loopback results, symptom (light good, link down), the one swap and where, link state after, root-cause panel, and the do-not-touch warning. Seven fields, and the next tech knows the pair is correct as it stands.

See RackReady   Free sample   Fiber no-link checklist

Support: support@hasherventures.com

Free tool: turn your polarity fix into a structured ticket note with the RackReady Ticket Builder.