Zen XT8: New DOCSIS 3.1 modem, slow ethernet speed

[JerseyMike]

So my old Motorola MB8600 modem finally died, so I replaced with a brand new Motorola MB8611 that has the 2.5GB port on it. I have Comcast's 1Gig service. Previously with my old modem connected to my Asus Zen Wifi XT8, I consistently got ethernet speeds in the mid to high 900s (tested on Ookla's site and dslreports). However, since getting the new modem, I've noticed my ethernet speeds on the LAN side get cut by about 30%. When running the speed test on the router itself (router <--> modem) , it consistently shows speeds of over 1G:

However, when running the same speed test (Ookla) to the same server from a PC that is wired to one of the LAN ports, the speed is significantly reduced:

I've done the usual steps (factory reset, make sure firmware is updated to latest, QoS turned off, No port forwarding, etc), but I still can't wrap my head around what the problem could be and can't seem to get back to the 900s that I used to get with the old modem. The only thing that has changed was the modem being replaced by a faster one. Are there any known issues using 2.5GB wan port that causes this router to choke?

 

[L&LD]

Are the LAN ports 1GbE? Is the testing PC using their own 1GbE port?

Many ISP's states that their >1Gbps service is only fully utilized with two or more client devices being used concurrently.

Did you reboot the network lately? Including the testing clients? And then let the network settle for a good 10 - 15 minutes or more?

 

[JerseyMike]

Are the LAN ports 1GbE? Is the testing PC using their own 1GbE port?

Yes.

Many ISP's states that their >1Gbps service is only fully utilized with two or more client devices being used concurrently.

I've got several client devices on the LAN port. In addition, when running the speed test between the router and the modem, I'm getting full gig (actually over a gig...Comcast's 1gig service is actually 1.2 gig).

Did you reboot the network lately? Including the testing clients? And then let the network settle for a good 10 - 15 minutes or more?

Yes, multiple times.

 

[SactoJS]

So my old Motorola MB8600 modem finally died, so I replaced with a brand new Motorola MB8611 that has the 2.5GB port on it. I have Comcast's 1Gig service. Previously with my old modem connected to my Asus Zen Wifi XT8, I consistently got ethernet speeds in the mid to high 900s (tested on Ookla's site and dslreports). However, since getting the new modem, I've noticed my ethernet speeds on the LAN side get cut by about 30%. When running the speed test on the router itself (router <--> modem) , it consistently shows speeds of over 1G:


View attachment 35954

However, when running the same speed test (Ookla) to the same server from a PC that is wired to one of the LAN ports, the speed is significantly reduced:

View attachment 35955

I've done the usual steps (factory reset, make sure firmware is updated to latest, QoS turned off, No port forwarding, etc), but I still can't wrap my head around what the problem could be and can't seem to get back to the 900s that I used to get with the old modem. The only thing that has changed was the modem being replaced by a faster one. Are there any known issues using 2.5GB wan port that causes this router to choke?

I don't have the 'Internet Speed' tab at that screen on mine. I am running FW 3.0.0.4.386_25790 - is that something they added with newer FW?