Auto-MDI/MDI-X on RJ45 Jacks: Crossover Cables Are Obsolete — VOOHU

TL;DR: With Gigabit-or-faster gear you don't need a crossover cable — the PHY behind the RJ45 jack (母座) handles it.

• MDI (PC/router): TX on pins 1&2, RX on 3&6.
• MDI-X (switch): swaps internally — RX on 1&2, TX on 3&6 — so straight-through cables work.
• Auto-MDI/MDI-X: the port detects its partner and flips automatically.
• Mandatory in 1000BASE-T (IEEE 802.3 Clause 40); standard on 2.5G/5G/10GBASE-T.
• The jack doesn't cross over — it routes all four pairs to the PHY. Pick the socket for signal integrity + magnetics, not polarity.

Full guide: https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/auto-mdi-x-rj45-jack-crossover.html