RT-N66U LAN speed decrease

[IYellowSnowI]

I've been having some problems over the past couple of months with my router cutting LAN speeds in half. I have a computer (and other devices) connected to an 8 port TRENDnet gigabit switch that's connected to the router. When a connection is gigabit, it is shown on the switch as a green light, and if it's 100 Mbps, it's a yellow light. Everything I have connected to the switch is via cat 6a cords and all the devices have gigabit connections, and the switch is connected to the router with a 100ft cat 6a cable.

Everything works great most of the time, but here and there, the connection will drop. The green lights (denoting speed and power of each cable) will stop just for the connection from the switch to the router, and then when it comes back on, it will be yellow and the speed between devices on this switch and other devices connected to the router will be cut in half - if that makes sense. Basically, the connection from the router drops, and when it comes back, it's showing a 100 Mbps speed on the switch, and if I transfer files from/to a computer connected to the switch and one that is connected to the router, it's much slower than it regularly is. Also, my internet speed (through ISP) is usually ~60Mbps down, but after this happens, it will stay in the 30-32 range only for devices connected to the switch.

The only way I have been able to "fix this" has been to go and unplug the Ethernet cord connecting to the router and plug it into a different port on the router. I have 3 cords connected to the router LAN ports, so I have 1 port free that I can switch it to. However, when I do this, it will disconnect every device connected via LAN for a moment (doesn't matter which port on the router). If I plug in a different device (not the switch) to the port I just moved the switch from, that device will also see the same decrease in speed.

The router is running Asuswrt-Merlin 374.39. The only "customization" I have done to it is put in DHCP manual entries, setup the WiFi networks, and setup the Samba Share. I have a external HD connected to one of the USB ports on the router (it's externally powered), maybe it could have something to do with this?

Anyone have any ideas on what I should do or what's causing this?

 

[CaptainSTX]

Things to check:

Be sure your network only has a single DHCP server. Be sure if you have a modem it isn't also assigning DHCP. Check the same thing with the switch that it is really only a switch.

Verify how connections are made:

WAN port modem - WAN Port router
LAN Port router - LAN Port Switch

If this doesn't solve the problem you are going to have to check each component and cable to isolate which one is causing your problem. Remove the switch and make connection directly to router and see if this eliminates the problem. Try different cables. Be suspicious of longer cables and test/bypass them first.

Remove most or all but one device from network. A bad adapter could be causing the problem.

Without test gear there isn't any quick way to isolate the problem other than trial and error.