Question ASUS AC-88U Failover / TL-ER5120 V1.6 Download Speed Issues...

May 23, 2019
Hello All...

I am beating my head against the wall trying to figure this one out. I had purchased a TP Link TL-ER5120 a few years back & ended up not using it due to a move.

Fast forward 5 years and my apartment complex provides me with internet, but when it goes out, it is out for a few days at a time. I'm going back to school (online) so I figured I would get MediaCom (only choice available) as a backup.

I have an ASUS AC-88U router that I was able to get both of the connections to work on -partially- as a fail-over. The APT (Primary) will fail over to the MediaCom (Secondary) after the allotted time of 60 seconds undetected, however it won't "FailBack" after detecting the APT internet for 30 seconds. (Tested this by unplugging, watching it switch from Cold/Standby to Active, see the Primary come back on as Hot/Standby, but it won't switch back). Not a huge issue, as I could manually reboot the router if this happens, but not working as intended. Plus the Network Status says it's disconnected on the GUI, even though it is working.

So, I thought, "Hey, let's get out this old TP Link TL-ER5120" and make it work, since it does load balancing/failover/etc... So, as it's only a V1.6, I downloaded the latest official firmware for it and loaded it up.

I got both of the WAN ports configured (and the LAN) and I can get on it, I see both connections (APT is Pri / MediaCom is Secondary), all looks good.

So here's where it gets weird. On their own (not plugged into TP Link) either directly to the computer -or- to the ASUS router, I get around 900Mbps/940Mbps (APT Primary) and around 230Mbps/30Mbps (MediaCom Secondary)...

When I plug them both into the TP Link, I get around 77Mbps/98Mbps to 130Mbps/129Mbps. So, for troubleshooting, I unplug the MediaCom and only have the Primary APT provided thinking it was some issue between the two, and with only the Primary WAN running I get 200Mbps/185Mbps.. Plugged them in individually to the ASUS in the main room, both perform fine at 900/900 and 225/29... Thinking maybe it was my Ethernet cable going to the back office, I swapped it out to no avail. Plugged the same cable directly from the TP Link into my computer and viola! Back to 900/900. Immediately plug it back into the TP Link and right back to 205/180 this time.

Checked the port status, etc. Everything is set to auto negotiate, reading gigabit ports, no errors. From what I'm reading, I can't upgrade the firmware past what I have (as the V2/V3/V4 versions won't work on my V1.6) and even looked at DD-WRT to try to find something to use (I'm using DD-WRT Merlin on my ASUS). I just have no idea why the 1gb ports are running so slow. i.e. If it's a firmware issue, that's a poor product or if I'm just not checking/unchecking a setting after reading through the user's manual a half a dozen times.

Sorry for the length of the post, hopefully somebody has some experience with either getting the ASUS AC-88U to optimize/failover properly or the TP-Link to properly utilize all available bandwidth.

Any/All help is much appreciated!
The problem with high speed internet connections is the CPU in most routers is too small to keep up. To solve this they have added features that allow the nat to bypass the main cpu. It varies a bit between vendors but most do this in special hardware on the chip.

Older routers did not have this because it was not common to have over 200mbps internet. Not sure if that is the problem with you old router or not.

The problem of primary/backup has been a issue since the beginning of internet. What is considered down 10% loss 50% 100%? and how do you detect this. It should see you unplug the cable though. This is why routing protocols were invented and what is one of the key differences between a actual router and the boxes sold to end consumers. Something like OSPF or BGP Is consently sending small messages between the routers to decide when to switch and what is the best path. Be nice if the ISP offered this service to smaller customers.

In any case I would go to and ask about the asus merlin function. It maybe a known bug in asus or it could be a issue with merlin. The guy "merlin" does his support on that forum so you may even get a answer from him.

It should work without a reboot. Unplugging the able should force a switch over
May 23, 2019
Thanks for the info, I thought it might be a combination of age (with the speed issue) even though it's supposed to be a full 1Gb/1Gb per port on the TPLink...

As for the ASUS AC-88U, I am manually unplugging. Specifically I've got the options to set it to frequency / amount of loss and either DNS Resolve / Ping...

Currently the threshold is 5 second / Ping (Amazon) and at a loss of 6x (i.e. 30 seconds) to switch over to Secondary, with a failback of detecting the Primary as good again for 3x (i.e. 15 seconds).

Manually unplugging to simulate the loss of Primary does work and it fails over properly ( Yay!), however, after letting it run like that for tests of 1minute / 3 minutes / 5 minutes and then manually plugging Primary back in to simulate restoration, it will detect the Primary again, but it won't failback to Primary.

I am aware of this by
A. Keeping a constant ping going to Amazon (Primary is about 13ms and Secondary is 30ms) and I see the 3 dropped packets and transition from 13ms to 30ms for the initial switch and upon plugging the Primary back in, it will drop a few packets here and there, but keeps going back to the 30ms. and
B. The ASUS AC-88U GUI will show the Primary going from being 'Unplugged' status to Cold Standby, then Hot Standby and cycle through, but never transition back.

I'll give it a longer test later as I'm playing around with the numbers and see if I can get it to properly fail back. As I was saying, not a huge issue, as I can always manually reboot the router and it 90% of the time it switches back to Primary.

Also, I did one last test after posting this and before bed last night with the setup. I plugged the APT Primary directly into the ASUS AC-88U and -WAS- able to get the load balancing to work now too. Set them up as a 5:1 ratio (as the Pri is 900/900 and Sec is 220/35ish) and it seems to be working other than an occasional hiccup on resolving a website (I see a handshake notification in the browser's lower left corner). Put in manual DNS servers thinking it might be a conflict with 2 different uplink's different DNS servers to see if that helps. Speedtest is showing the full speed of the Primary, so as long as I don't get the random 30 second resolves on the web pages (hopefully fixed by manually specifying the DNS) we should be good to go. Just scrap the old TP Link, lol..

Thanks for the help and I'll post results when I get back today if that solved the random laggy webpage bit