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ER605 L2TP clients disconnect when doing upload test (Speedtest.net)

ER605 L2TP clients disconnect when doing upload test (Speedtest.net)

ER605 L2TP clients disconnect when doing upload test (Speedtest.net)
ER605 L2TP clients disconnect when doing upload test (Speedtest.net)
Yesterday - last edited 15 hours ago
Tags: #VPN #WAN Setup #MTU
Model: ER605 (TL-R605)  
Hardware Version: V2
Firmware Version: 2.3.3 Build 20251029 Rel.18054

Hardware version is 2.30 (UN) (europe) 

 

ISP: PPPoE, MTU 1492 , 1/1 Gigabit Fiber 

 

Clients tested: iPhone 16 Pro and Windows 11

 

I've setup an Encrypted L2TP server that i've successfully connected to with the two mentioned clients. 

 

When i do a speedtest.net the client will download at 100~180 Mbit/s 

 

When the upload starts i get around 50-80 Mbit/s for about 1 second wherafter the L2TP connection drops. 

 

I've looked at the CPU graph while testing and it reaches 50-70% so it shouldn't be the CPU limitation of the router.. 

 

ChatGPT and Claude both suggested that the problem is MTU. Pinging 8.8.8.8 was only successful at 1344 byte 

So i tried configuring the MTU size on the WAN to 1360 (suggested by Google-Gemini when ping happens at 1344) 

 

That didn't help... 

 

NB: Tomorrow i'm getting a new ISP that uses DHCP instead of PPPoE - so that might help... 

 

NB: The system log ( All levels ) dont report any problems 

 

Any suggestions ? 

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#1
1 Accepted Solution
Re:ER605 L2TP clients disconnect when doing upload test (Speedtest.net) -Solution
16 hours ago - last edited 15 hours ago

  @Nathan-TP 

 

Hi again

 

I've run some more tests and given up on Speedtest.net 

 

I can upload at 50Mbit from phone via L2TP to a server on the network just fine.. so the problem seems to be Speedtest.net 

 

Comparably i tried again on Windows 11 and here it works fine in all regards. 

 

i think ill close it here :) 

 

Thanks !

Recommended Solution
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#4
3 Reply
Re:ER605 L2TP clients disconnect when doing upload test (Speedtest.net)
Yesterday

 Hi,@MartinKirk 
Thank you for posting on our business forum.

We would like to recommend you try the following troubleshooting steps first:
1.Please adjust the MTU value of your related configuration to 1400.
2.Navigate to Network > WAN, select your PPPoE WAN connection, then enter Advanced Settings to locate the MSS Clamping option. Kindly enable it and set the value to 1350. After completing this setup, please ensure the same MSS Clamping value is applied to your L2TP VPN server configuration.
3.Please repeat the test with an alternative client device.

If the issue still persists after going through the steps above, could you please kindly share the following additional information with us to facilitate further investigation:
1.Are you managing this ER605 device via an Omada Controller?
2.Is this issue reproducible across all of your different tested clients
3.Have you tried testing with the ER605 bypassed, or performing the test within the local network of another router for comparison?
4.Has the issue been resolved after you switched to your new ISP?
Thank you for your cooperation.

 

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#2
Re:ER605 L2TP clients disconnect when doing upload test (Speedtest.net)
23 hours ago

  @Nathan-TP 

 

Hi , the new ISP is using DHCP (Static IP) without PPPoE, MTU is now 1500 (default) 

With the deafult settings i tried L2TP VPN 

 

Windows 11 client doesn't disconnect now and i get 140/110 Mbit (which is nice)!: 

 

 

iPhone still disconnects when uploading :( 

 

Shouldn't the L2TP procol negotiate an MTU which is lower than the WAN in ALL cases ? 

 

I dont understand why lowering the WAN MTU helps, if the L2TP client works in the same size - since L2TP overhead will be added on top and therefore fail to get through the WAN MTU ? -- or am i misunderstanding how it functions ? 

 

I would expect the L2TP VPN server to work within the limits of the WAN setup.. 

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#3
Re:ER605 L2TP clients disconnect when doing upload test (Speedtest.net) -Solution
16 hours ago - last edited 15 hours ago

  @Nathan-TP 

 

Hi again

 

I've run some more tests and given up on Speedtest.net 

 

I can upload at 50Mbit from phone via L2TP to a server on the network just fine.. so the problem seems to be Speedtest.net 

 

Comparably i tried again on Windows 11 and here it works fine in all regards. 

 

i think ill close it here :) 

 

Thanks !

Recommended Solution
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1
#4

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