Arris S34 2.5Gb port speed issue

I’m going to ask the obvious question, because it’s the one thing that you didn‘t report:  do you actually have a 2.5 Gb/s line from xfinity?  Also, that theoretical 2.5Gb/s speed is assuming that there’s only one device that will use it - the more equipment you connect to the network means the less actual speed you’ll achieve.  Think of that cable as a garden hose.  Works fine when you’re washing the car, but, run a line from there to the dishwasher, then another to the washing machine, shower, toilet, sprinklers, etc., and the pressure that initially came out of that hose is now just a trickle.

Hi, thanks for the reply.  No, I do not have that speed provisioned from my ISP, but I’m aware of the technicalities of networking (I’ve spent a couple of decades in the field).  Maybe you missed the part where I get pretty-much maxed-out 1Gb speeds on a 1Gb link, but only 140Mb/s on a 2.5Gb link - only when connected to a 2.5Gb NIC.  Connecting the modem’s 2.5Gb port to a 2.5Gb NIC drops the speed substantially.  I do appreciate the response, but this is a repeatable issue and it appears I’m not alone.

Disclaimer: i have an S33 v2 and have no knowledge of the S34, however;

I’m not sure that you answered the acpiper’s main question. Which was, what speed is you ISP providing? I believe that Xfinity offer line speeds up to 2Gb, but in my area of the world (Pac NW),  they currently only offer me up to 1.2 Gb. So you might want to check that your actual line speed is, as expected. Just a thought, hope this helps. 👍

Happy to provide any details needed, as long as it doesn’t cause more confusion.  I’m currently paying for the 800Mb tier from my ISP, but I believe that’s irrelevant and I’ll explain why.

I know I’m not going to get any more speed out of my ISP without paying for a faster plan.  The reason I gave the 940Mb figures earlier is because I’m lucky enough to have excellent signal strengths and good quality cabling, plus maybe a little over-provisioning in my neighborhood but I should not expect more than 800Mb.  The S34 modem was specifically listed as being fully-compatible with Xfinity, DOCSIS 3.1 compliant, I know I’m on the next-gen network at Xfinity as I have an upload speed of 150Mb - I had the whole rigmarole of being capped at 40Mb until I said the right thing to the right person to get my billing codes updated, after which I got the full expected upload bandwidth and I have no issues getting the 800Mb speed I am paying for.

Let’s go back to when I had 400Mb, or even 200Mb service from my ISP - I did not expect 1000Mb just because my home LAN can talk “up to” 1000Mb - I know my internet traffic can only go as fast as it’s provisioned.  Let’s say my modem had a 10Gb port, and it’s connected to a 10Gb LAN - I know I’m still not going to get any more than 800Mb of internet traffic routed across that 10Gb network.

I’ve been enjoying my 800Mb of ISP bandwidth on my 1000Mb LAN, and now I want to upgrade my LAN to 2.5Gb - which I have done, but the one limiting factor is the port speed on my modem, everything is only as fast as the slowest link, right?  So I change from the 1Gb port to the 2.5Gb port so that my whole LAN can now talk “up to” 2500Mb, even though I know I’m still only going to get 800Mb of bandwidth from the WAN.

Using the 1Gb port on the S34 connected to my 1Gb LAN I get my 800Mb bandwith.  Using the 1Gb port on the S34 connected to a 2.5Gb LAN I get the same 800Mb.  Using the 2.5Gb port on the S34 connected to a 2.5Gb NIC I should still get 800Mb of bandwidth from my ISP, right?  Nope, that’s where the problem lies - the speed is dropping to less than 20% of what it should be - but only when the link speed is 2.5Gb at both ends.  Usng the 2.5Gb port but negotiating at 1Gb works fine.

I at first thought maybe the S34 2.5Gb port was auto-negotiating down to 100Mb speeds due to some kind of incompatability with 2.5Gb NICs (which is why I tried several), but the upstream bandwidth doesn’t change from the 150Mb that I get consistently which is well above the 100Mb limit so it does not appear to be that.

The whole reason for buying this specific S34 modem was the fact that it has a 2.5Gb port, and I know I want to upgrade my plan to the 1.2Gb or maybe even the 2Gb tier at some point in the near future - but even at this current lower speed tier, I still contend there’s something wrong with the S34.  Let’s say I upgrade to the 2Gb tier tomorrow - it appears my speed would be throttled to either 150Mb or less than 20% of what it should be when using the 2.5Gb port connected to a 2.5Gb device.

Thanks for the conversation, I hope I’m articulating the issue in an understandable manner.

you say you’re connecting to a 2.5g nic. Which one? 

And are you running a pfsense/diy router? or what is acting as a router? 

I tried this Intel I-226V chipset-based card, this Realtek RTL8125BG based-card, and finally this I-226V card (that I suspect is the exact same card as the first, just under a different name).

All of these were installed in my pfSense (v2.7.2 CE) router, which is a Dell Optiplex O755 with a Core 2 Duo E8400 3GHz CPU and 4Gb of RAM, which doesn’t break a sweat.  All 3 NICs showed as negotiating at 2500Mbs without any issue, I tried both autodetect and fixing at 2500Mb and while it didn’t appear that I needed to install the Realtek driver package for pfSense I did anyway, it didn’t change anything.

Again, please remember that I have no issues with the S34 2.5Gb port when connected to a 1Gb NIC - only when it’s connected to a 2.5Gb NIC.

do you have a different 2.5gig device (not in the pfsense box) you can try? 

I’ve seen plenty of posts on reddit of pfsense issues on 2.5g. 

Yes, I tried both this and this NIC in two desktops that I connected directly to the modem (bypassing the pfSense router to rule out any issues there).  I even tried this USB to 2.5Gb Ethernet adapter for grins, to rule out any possible PCIe issues.  I tried both this and this 2.5Gb switch, because they had 10Gb SFP+ uplink ports to connect to my Mikrotik 10Gb switch for the rest of my 10Gb machines.

I’d open a support case with arris. Could be a defective port

I contacted them at the only email address I could find, consumersupport@commscope.com 5 days ago and I’ve not yet heard anything back.  I’m glad we agree that we suspect the 2.5Gb port on the S34 as having an issue.