TL;DR: Gigabit ports auto-swap TX/RX, so crossover cables are obsolete. The RJ45 jack doesn't do the swap — the PHY does. Just wire all four pairs to it.
The two port roles
- MDI (PC/router): TX on pins 1-2, RX on pins 3-6.
- MDI-X (switch/hub): the reverse — RX on 1-2, TX on 3-6.
- Straight cable + one of each = link works. Two of the same = you historically needed a crossover cable.
What Auto-MDI/MDI-X changes
- The PHY flips its own MDI/MDI-X state during auto-negotiation (IEEE 802.3 Clause 28).
- A pseudo-random seed stops both ends flipping at once.
- Optional at 10/100; mandatory in 1000BASE-T (Clause 40); standard in 2.5G/5G/10GBASE-T.
- Gigabit uses all 4 pairs, so auto-crossover also fixes pair swaps + polarity, so any cable links.
Where the RJ45 jack fits
- The jack is passive: 8 contacts to 4 pairs to magnetics to PHY. The PHY decides MDI/MDI-X.
- Route all four pairs (1-2, 3-6, 4-5, 7-8) — a 2-pair 10/100 socket blocks Gigabit auto-crossover.
- Terminate to T568A/B cleanly — auto-MDI-X will not rescue a split pair or mis-wired jack.
Gotchas
- Hard-forcing speed/duplex can disable auto-negotiation, which disables auto-MDI-X. Leave auto-neg on.
- If a certified cable fails on one port only, suspect the jack, not the crossover logic.
VOOHU's RJ45 + SYT integrated-magnetics jacks carry all four pairs from 10/100M to 10G Base-T. Full write-up with pin tables + a support-by-standard matrix: https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/auto-mdi-x-rj45-jack-crossover.html
Top comments (0)