WRT1900ACS with 2.5g modem?

I needed to upgrade my modem from a DOCSIS 3 model to a DOCSIS 3.1 modem. The new modem has a 2.5G ethernet port.

When I connect the gigabit NIC in my PC directly to the modem, I can connect to the internet.

My router is an old Linksys WRT1900ACS v2 which has gigabit WAN and LAN ports.

When I connect the router (with stock Linksys firmware) to the modem, it fails to connect to the internet. It seems that it cannot negotiate a connection with the 2.5g ethernet jack on the modem. I thought that was odd given that the gigabit NIC in my PC can. I had assumed that it would be backward compatible. Looking at the manual for the modem, it describes the ethernet jack as 10/100/2500 which seems to imply that it doesn't do 1000. But I go back to the fact that the NIC in my PC works.

Is there any chance that OpenWRT would be able to get the router to successfully negotiate a connection with the modem?

If so, what settings should I look at? Are there any particular packages that I'd need to add?

Any suggestions or guidance would be greatly appreciated.

If there is no way to get this working, can you suggest an OpenWRT compatible router with 2.5g WAN port? I'd really like for it to have a USB 3 port and would like to keep price under $200US. Closer to $150 would be better.

There is a chance.
Here was someone with a WRT1900AC running OpenWrt, and seems not having any issues connecting to a modem with a 2.5G ethernet port.
https://forum.level1techs.com/t/isp-upgraded-network-to-2gbps-time-to-update-hardware/178135

Try it? If you are interested in running OpenWrt, you can on your current router.

If you are looking to upgrade, GL-MT6000 would be a good contender.
https://openwrt.org/toh/gl.inet/gl-mt6000

It's about $160 right now.

What's the internet speed you have?

I fully expect your DOCSIS 3.1 would be fully backward compatible to 10/100/1000 (there is very little equipment remaining requiring 10/100).

Your 1900ACS is still a very viable box running on wire and I expect to be running mine until fibre comes in 4-5 years down the line. As @dnd says, try OpenWrt. It will still give you ~500Mbps if you have it.

OOTB, it’s pretty well plug & play, and the dual partition feature let’s you easily return to OEM firmware just by booting to the alternate partition.

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I went ahead and installed OpenWRT. It was a surprisingly simple process, really no different than installing stock firmware.

But the router will still not negotiate a connection with the modem.

The the status overview shows no IPv4 active leases. Is that significant?

I really don't know what to look at to diagnose the problem. Is there anything I should check in the system log? Any terminal commands I should try running?

You can post the system log here, maybe someone can spot something weird.
You can also try using ethtool to get more detailed info about the state of the wan device.

No, it's the lan side of things, where your router is the DHCP server. Your problem is on the wan side.

Edit: Just for laughs, can you try putting a switch between the router and the modem?

I do happen to have a gigabit switch so I just tried that. It didn't make a noticeable difference.

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I do not think the issue is in the ethernet speed. It just does not make any sense that the modem can do 2500/100/10 but not 1000.

How did you configure your PC to stablish a connection? Does your ISP limit connections by MAC address?

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That's a good point!
I have an issue with my fiber ONT, if I switch routers around, they don't get a connection.
I have to completely power cycle the whole setup, then it works.

Maybe your new modem does the same?

I simply plugged one end of an ethernet cable into the PC NIC and the other end into the jack on the modem. I didn't do anything else.

I can't believe it.

I just unplugged the modem for a minute or two then plugged it back in. Watched it slowly turn on each of its lights.

Bam! It's working now!

I've been doing cable swaps left and right. And I've power cycled the router many times. But it must not have occurred to me to power cycle the modem.

Thanks so much!

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Yep. DOCSIS will do this to you every time. You’ll need to repeat the process if you switch partitions to go back to OEM firmware.

I'll keep that in mind. I am going to play with OpenWRT for a while; I've always wanted to try it out.

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