Low LAN to WAN throughput on Asus RT-AC5300

[ai4snb]

I purchased the router some 50 days ago, and until recently it worked great. I have a 1GBps AT&T fiber connection and was getting 900+MBps on my wired devices.
Sometime in the past week or so, the LAN to WAN throughput has dropped to 50MBps down and 100MBps up. I made no changes, just added some more devices on the network. Connecting directly to AT&T's router gives me the full 1GBps, so the problem seems to be with the Asus.
I am running AsusWRT-Merlin 384.7_2. No QoS or filters enabled, just a bunch of static IPs and 4 measly port forwarding rules, all of which were there before this slowdown started to happen.
I tried making most of the DHCP leases static, since I read somewhere that it might impact performance, but that didn't seem to have any effect.
Any idea what could be causing this?

 

[wiz]

What happens if you reset to factory default and configure from scratch?

 

[ai4snb]

I tried both "Restore" and "Initialize" under "Factory Default", to no effect. I even tried having just one client on a wired connection, with nothing else plugged in or connected via WiFi. The problem persists.

 

[wiz]

If you clone yr mac address from the pc you tried to connect straight for yr wan, does that help?

 

[ai4snb]

The router AT&T gave me (model 5268AC FXN) is worthless due to its horrible interface. I can connect my PC directly to it and it will give me the full 1GBps. I connected the Asus to it and set it to be the DMZ host in the AT&T router's interface. There is nothing else connected to the AT&T router. Up until last week, anything connected to the Asus was getting full speed as well. I can't take off the DMZ for the Asus because the AT&T router then throws a "router behind router" error and refuses to connect. I have them on different subnets (AT&T has 192.168.0.x and Asus has 192.168.1.x).

 

[ai4snb]

Ok, this gets weirder: following your suggestion to clone the PC's MAC takes my speeds to... 250MBps on speedtest.net, which makes no sense whatsoever, as switching back to plugging just the PC into the AT&T router, with the same (cloned) MAC gives me the expected 1GBps.
However, I seem to get 1GBps through the Asus on dslreports.com, so your suggestion did solve my problem. I still wonder why all this would happen - perhaps it has to do with speedtest.net using one server and dslreports.com using 2+ servers from different regions?
I also noticed that speedtest.net completely saturated 1 of the 2 cores of the Asus (the other was mostly idle), while dslreports.com inches toward using the entire core but only gets there when uploading, and after several seconds of sustained uploading. Perhaps there's a lesson to be learned in multithreaded/core management.

 

[joe scian]

Ok, this gets weirder: following your suggestion to clone the PC's MAC takes my speeds to... 250MBps on speedtest.net, which makes no sense whatsoever, as switching back to plugging just the PC into the AT&T router, with the same (cloned) MAC gives me the expected 1GBps.
However, I seem to get 1GBps through the Asus on dslreports.com, so your suggestion did solve my problem. I still wonder why all this would happen - perhaps it has to do with speedtest.net using one server and dslreports.com using 2+ servers from different regions?
I also noticed that speedtest.net completely saturated 1 of the 2 cores of the Asus (the other was mostly idle), while dslreports.com inches toward using the entire core but only gets there when uploading, and after several seconds of sustained uploading. Perhaps there's a lesson to be learned in multithreaded/core management.

do you have NAT acceleration on?

 

[ai4snb]

It's set to Auto - "CTF (Cut Through Forwarding) is enabled". The only other option is "Disabled", there's no "On".
Without making any changes, now it seems speedtest.net is giving me 1Gbps as well. Not sure what happened, just hope it lasts.

 

[CaptainSTX]

I am curious about how your double routers are setup and configured. I have never heard of the first router in a two router setup caring about what is connected to it and if the double NAT was done correctly. Second having to put the double NATed router in the setup in the first router's DMZ is not usually/ ever required.

1. Is the Ethernet cable from the AT&T router connected from a LAN port to the WAN port of the ASUS router?

2. On the ASUS router did you set its WAN IP to automatic then if you want, using the the AT&T router assign the ASUS router an IP in the range 192.168.0.0/24? Also is the IP you assigned the ASUS's WAN port outside the range of the DHCP pool on the ASUS router?

3. On the ASUS did you assign its gateway as the LAN IP of the AT&T router?

 

[ai4snb]

Yes to all three. I misspoke, the AT&T router does not require me to DMZ the Asus, it just strongly suggests it. There is an option to disable the warning. When DMZ-ed, the Asus reports its WAN IP to be identical to the AT&T's. When not, it gets a 192.168.0.0/24 IP.
Using DMZ reliably makes the problem reappear, so it seems AT&T is throttling DMZed devices.

 

[CaptainSTX]

Yes to all three. I misspoke, the AT&T router does not require me to DMZ the Asus, it just strongly suggests it. There is an option to disable the warning. When DMZ-ed, the Asus reports its WAN IP to be identical to the AT&T's. When not, it gets a 192.168.0.0/24 IP.
Using DMZ reliably makes the problem reappear, so it seems AT&T is throttling DMZed devices.

OK. Thanks for answering my questions. I have run double NATed for most of the past ten years with three ISPs and never had similar problems.

 

[Cellborne]

I am so thankful that I found this forum. I have the same issue except with Asus software. For some reason when upgrading to the latest firmware my download speed slows to about 50 down and about 100 up. The same trick of putting in a mac address in the mac clone makes the router speed back up to full 1gb down/up. I also have a ATT router. I had just RMA'd my old router with the new one. Speed tested it and it was fine. For some stupid reason I decided to upgrade and it again slowed down. I was so sick to my stomach and was on the phone with Asus about to RMA this new router I just got when I found this fix. How this fixes both the merlin and the asus codes I have no idea. I am truly thankful I don't have to ship this router back for a second time. Big thanks to "wiz" and is suggestion. I am truly grateful!

 

[ai4snb]

Glad the solution worked for you as well.
It's no big mystery why this works for both Asus and Merlin's version: AT&T is now throttling DMZed devices. It can be something as simple as "if the MAC address I'm seeing is not from one of our routers, I'm going to throttle", which is what happens if the Asus router is set to be the DMZ host. I hope they reverse this decision, as now I have to rely on port forwarding to open ports for services I need.