TP-Link's wired advice: make sure the cable and the PC can match the Deco's LAN port speed, since a 100 Mbps port or cable caps results near 90 Mbps regardless of plan (skip this check if your plan is under 100 Mbps), and use Cat6 or better for 2.5 Gbps and up. Test in order: PC to modem for the baseline, then the same PC on the main Deco. For a wired backhaul between Decos, all units should show the same speed. Set the Decos up wirelessly first, then connect cables; the backhaul turns on by itself.
In Router mode, satellites plug into the main Deco's LAN port or a switch connected to it, never into the modem. Swap the cable, change port, and try a different Deco unit to find whether the fault follows the cable, port or unit.
The checks in more detail
If a switch sits between units and wired satellites drop, remove it for the test. Turn off QoS while testing, because it can reduce results. Check that a 2.5G or 10G port is actually connected to a device that supports it, and that the Deco is not negotiating 100 Mbps because of a damaged pair. Label your cables and note which port each one plugs into.
A cable that is slow in one run is often fine in another, and knowing which is which turns a guessing game into a two-minute check. Cheap flat cables and old Cat5 leftovers are the most common culprits. When you wire a home, make sure every run is tested end to end before the wall is closed, since a run that only negotiates 100 Mbps costs the same to install as a good one and is very hard to fix later.
Differential
Likely causes of “TP-Link Deco mesh wifi Ethernet link is limited to 100 Mbps or disconnects”, and how to tell them apart
- 1
A 100 Mbps cable or port
What separates it The wired speed caps near 90 Mbps because the cable or the port only runs at 100 Mbps
How to confirm Use a Cat6 cable and gigabit ports on both ends, and test the speed again
- 2
A damaged cable
What separates it The wired link is slow or drops because the cable is damaged
How to confirm Swap the cable for a different one, and check the speed once more
- 3
A satellite plugged into the modem
What separates it There is a loop or slow speed because a satellite is plugged into the modem
How to confirm Connect the satellite to the main Deco, and not to the modem
- 4
A switch limit
What separates it The wired satellites are slow because a switch in the path limits the speed
How to confirm Remove the switch as a test, and see whether the speed goes up
- 5
QoS on
What separates it The speed test reads low because QoS is switched on
How to confirm Turn QoS off in the Deco app, and run the speed test again
Before you touch a setting
Record this first — changing settings destroys the evidence
- The type of cable you use and its length
- Port speeds on device and Deco
- Speed at the modem and the main Deco
- Whether a switch is in the path
Try in this order
0 of 7 completedCheck the cable
ReversibleCheck the cable, using Cat6 for fast links, since older cables cap the speed, and then test again to see whether it helped.
Test at the modem
ReversibleTest at the modem, to get a baseline speed that shows what the provider is giving you.
Test at the main Deco
ReversibleTest at the main Deco with the same computer, and compare the result with the baseline.
Swap the port
ReversibleSwap the port and the cable, one at a time, to work out which one is causing the trouble.
Remove switches
ReversibleRemove switches from the path for the test, so that only the Deco is between the devices.
Turn off QoS
ReversibleTurn off QoS in the app, and then test again to see whether the result improves.
Compare units
ReversibleCompare the units, since with Ethernet backhaul every one of them should give a similar speed.
Model-specific
What is true of this model and not of its siblings
- BE85
- The BE85 has 2 x 10G and 2 x 2.5G ports.
- X55
- The X55 has gigabit Ethernet ports.
- Other models this page answers
- These steps also fit the Deco BE63 mesh, Deco XE75 mesh, Deco X55 mesh, Deco M5 mesh.
Stop boundary
When this stops being a self-service repair
- !If a known-good cable still caps at 100 Mbps, contact TP-Link support
- !If backhaul speeds differ between units, check the cabling
- !If the modem is slow, call the provider
Evidence ledger
Sources behind this answer
Testing correctly and isolating slow speeds.
Wired backhaul rules.
Mesh basics and wiring.
Height, interference and signal bars.
The company and its products.
Exact questions
Common questions
Why is my wired speed 90 Mbps?+
A 100 Mbps port or cable.
Where do satellites plug in?+
Into the main Deco's LAN port or a switch on it.
Can I plug into the modem?+
No, do not plug a satellite into the modem, since it creates a loop and slows things down.
Should all units match speed?+
Yes, with Ethernet backhaul all of the units should give about the same speed.
Does QoS matter?+
Yes, QoS matters, since it can reduce the results of a speed test.
How do I test the baseline?+
Connect a PC to the modem.
Can a switch slow it?+
A switch can slow the link, so test without it, and see whether the speed improves.
Do 10G ports need special cable?+
Cat6a or better is prudent for the 10G ports, so that the cable does not limit the speed.