Why an Ethernet Link Drops from 1 Gbps to 100 Mbps

About ten days after completing the bridge setup, both Ethernet and Wi-Fi clients became limited to roughly 100 Mbps. It initially looked like a Kablonet, Infinity 401 or Archer AX73 configuration problem. The actual cause was the short Ethernet cable between the modem and router.

Symptom: every device hits the same ceiling

Different clients approached the same limit even though the Internet package and router were capable of more. That was the useful clue: the problem was not one Wi-Fi client, but the shared uplink carrying every client's traffic.

On my Infinity 401 unit, the LAN indicator behaved as follows:

Green LAN LED  → Gigabit Ethernet
Orange LAN LED → 100 Mbps Ethernet

LED mappings can vary by model or firmware. The reliable measurement is the negotiated WAN link speed shown by the router.

Why did a damaged cable fall back instead of disconnecting?

1000BASE-T uses all four twisted pairs in an Ethernet cable. 100BASE-TX can operate using two pairs. If one conductor, pin or RJ45 contact fails, the devices may negotiate a lower link rate instead of losing the link completely:

1000 Mbps
   ↓ pair / contact fault
100 Mbps

That is why “the cable still works” does not prove that the cable is healthy.

Fast diagnostic sequence

RUNBOOK

Modem → router uplink checks

0 / 5 completed

1. 01Check the router's WAN link speedIf it shows 100 Mbps, investigate the physical link before the Internet package.
2. 02Record the modem LAN indicatorConfirm the colour mapping in the documentation for your own model.
3. 03Replace the uplink with a known-good Cat5e/Cat6 cableTest directly without wall sockets, couplers or adapters first.
4. 04Reseat both RJ45 endsA loose connector or poor contact can reduce the negotiated speed.
5. 05Recheck link speed, then Internet throughputVerify 1 Gbps negotiation before running a speed test.

How I verified the result

After replacing the short uplink cable, the connection negotiated at 1 Gbps again and measured approximately 196 Mbps downstream and 20 Mbps upstream. No ISP or router setting changed.

This is a physical-layer problem independent of the Infinity 401 Bridge Mode and Archer AX73 procedure, but it can look like a bridge failure because one cable carries the whole network.