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RB5009 2.5g Port Problem

Sat Feb 12, 2022 4:22 am

I'm experiencing slow upload speeds when connecting my computer to the 2.5g port operating at 2.5g. If I remove the 2.5g full advertisement from the ether1 interface (forcing the computer to negotiate to 1g full) I do not experience the slow upload speed.

My Setup:
I have my internet on a 1g port (ether8) and I have the 2.5g port connected to my computer which has a built in Intel I225-V NIC which supports 2.5g full.
Internet -> (ether8 1g) RB5009 (ether1 2.5g) -> Computer with Intel I225-V supporting 2.5g

In my use case I have a 1g symmetric internet connection but I have my computer attached to the 2.5g port as a server so that it can access the internet and serve my 1g home network at the same time without losing bandwidth. All of my devices are connected to the RB5009, I do not have any other routers or switches in my setup (small home setup).

I have noticed that if I allow ether1 to advertise 2.5g, my computer will auto-negotiate to 2.5g (as expected) but my upload speed drops compared to that of a direct connection. If I remove the 2.5g full advertisement from ether1 the computer will auto-negotiate to 1g (as expected) and my upload will increase to expected

Here is my data:
Direct connection (no RB5009): 950/950 Mbps which is expected
2.5g Full: 950/600 Mbps which is 300 Mbps lower on upload
1g Full: 950/950 Mbps which is the same as a direct connection

I'm using Ookla for my speed test and these results are very consistent as I have swapped back and forth between these configurations several times. Whatever is happening appears to be systematic rather than random. There is something about the port operating at 2.5g and routing to my 1g WAN port that is causing a slow down only on upload (faster 2.5g interface sending).

I tried enabling RX and TX flow control on both the 2.5g port and the 1g WAN port but I got the same results.
I'm running RouterOS 7.1.2. I also tested 7.0.5 and 7.1.1 and all of the versions show the same slow down (I upgraded from 7.0.5 to see if a newer version would fix this issue which is why I tested older versions).

I have done a bunch of searching on this problem but most people are connecting their 2.5g port for WAN whereas I'm using it on my computer side and my WAN side is a 1g port (ether1). I don't have any 10g devices and I'm not using the SFP+ port.
 
marekm
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Joined: Tue Feb 01, 2011 11:27 pm

Re: RB5009 2.5g Port Problem

Mon Feb 21, 2022 1:05 pm

This may be the classic issue with bursty traffic overflowing a too small queue resulting in packet loss - can happen in any device where traffic enters the device through a larger pipe and leaves through a smaller one. Happens in many Ethernet switches with 10G port in -> 1G port out, or 1G port in -> 100M port out, too. The better of these devices have larger queues, sometimes of configurable size. Too big queue is bad too (bufferbloat), but with the high speeds we usually have today, it's often too small by default.
 
SergeP
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Joined: Sat Jan 15, 2022 7:53 am

Re: RB5009 2.5g Port Problem

Fri Mar 11, 2022 4:21 am

According to the logic of the person who answered the question, if you have a 24 1G port switch with 10G uplink port, your uplink speed will be limited to 1G.
In the old days that kind of devices were named "the HUB" - one connection at a time.
Normal "switch" will operate on 2.5Gb speed if one computer connected and negotiated 2.5G, full duplex and three others connected to three different 1Gb ports, negotiated 1G, full duplex and you downloading or uploading files from/to first computer to/from three others at the same time. My RB5009 behaves even worse: if I advertise 2.5G on ether1, the combined download speed ~200 - 300 Mb, and ~930 when I remove 2.5G advertisement.
(It's hard to be an alfa hardware tester....)

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