Slow/capped(?) upload speed behind vpn (router in bridge mode) - PPPoE ISP

Dear all,

Since switching our ISP (now fiber instead of copper) three weeks ago we experience a much too slow upload speed of only constant 25 Mbits/s where it should be close to 1 GBit/s. DS is as expected around 900 Mbits/s. All direct internet access measurements, not through any VPN. I am dead sure it's too slow as our cloud backups started to overlap due to slow US.

Setup:
  • ISP fiber connection with 1 Gbit US and DS. 
  • Fixed IP using PPPoE (dynamic DHCP, not MAC based)
  • Router Zyxel XGSPON Voip IAD (AX7501-B0) provided by the ISP and in pre-configured BRIDGE mode with VLAN 11 set AND a Zyxel VPN 100 (with default rules)
  • Firmware of the VPN 100 up to date (5.32).
If I measure speed directly on the router then US is as expected.
  • Whether I implement VLAN on the router or on the VPN does not change anything.
  • Changing MTU on the interface to avoid potential fragmentation does not change 'anything'.
  • With the old ISP we had US of around 100 Mbit/s which was correct back then.
Any ideas why our US is so extremely slow? Is it possible that it gets capped because I am running the router in bridge mode and connect using PPPoE on the VPN, instead of earlier MAC based fixed IP?

PS: I will in the future try to connect a fiber cable directly to the VPN using the (hopefully) correct tranceiver, but for now I would be happy to get this resolved first of all. It's much more important to me.

Any ideas greatly appreciated!
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All Replies

  • mMontana
    mMontana Posts: 1,088
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    CPU of VPN100 is 100% when connected?
  • Hello mMontana and thanks for your reply. No, CPU usage of the VPN is below 10 % and has never really gotten higher than 10 % as per the day chart I am looking at now. Also RAM usage is of no issue here. I will have to check the router but do not have those credentials at hand, will check tomorrow. thanks for the suggestion though, appreciated!
  • mMontana
    mMontana Posts: 1,088
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    PPPoE and WAN interfaces have setup about interface speed. Are they setup for allow 1GBPS?
  • jasailafan
    jasailafan Posts: 172
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    You can check the port speed of the interface.

  • Gothmog
    Gothmog Posts: 11
    First Comment
    edited November 2022
    Dear all,

    This is all default, WAN1 is on Auto Negotiate and Status is 100M/Full which is correct. 


    Here a speedtest of the lan showing way too slow upspeed. The graph always look similiar, it peaks at the beginning an then gets throttled in one way or the other. I have also tried overwritting the interface its MAC address with that of the router, but I couldn't even get internet running like that. Outgoing and incoming bandwith on all interface is set at default values of 1048576 Kbps. I might need to add, that when we used a 1000/100 copper based ISP everything worked fine. The only thing that changed is the router and the connection type (now PPPoE with autoretrieval of static external IP address, before MAC based with static external IP address).


  • jasailafan
    jasailafan Posts: 172
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     Master Member
    Seems there is compatible issue between the router and VPN100. Try to connect a Gigabit unmanaged switch between the router and VPN100 and check the download speed again. 
  • Hello jasailafan and thanks for your somehow funny proposal. I tried this just now, but it doesnt make any difference, here a new screenshot. it's the upspeed thats the problem. and one would expect that the the zyxel devices would be compatible with each other. but well... tomorrow, a technician will replace the router, maybe its the hardware.


  • mMontana
    mMontana Posts: 1,088
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    Sorry for being picky... Interface is not the port.
    Interface can be Wireless Lan, PPPoE connection, vLAN, and some other things. The port is the phisical port of the device, so i'm glad that your device can negotiate 1000MBPS, so the port won't limit the interface.
    This is the wan interface assigned to port P2 on USG Flex 50. The router connected have access to ADSL which negotiate 10798 kbps download and 1078 upload. According to these speeds, I changed the interface configuration. Firewall cannot access to negotiation data, so i limited the speed according to the ISP connection available. This avoids any kind of "error" for lack of transport capacity of the connection.
    Egress = upload
    Ingress = download
    I missed that correspondence so many times (with angry customers for slow internet) that now i cannot make that mistake again.

  • Hello mMontana,

    My interface is a PPPoE connection, the necessary VLAN (11) is set on the router (I have had it set on the VPN before, both work, but there is no difference to the too slow upspeed).

    The physical WAN port used is number two and these are the WAN settings. The numbers differ significantly from yours. However, in my understading, 1048576 Kbps is a correct value for appx. 1Gbit/s. I have both 1Gbit/s up and downspeed on the router. And my values seem to be correct for that.


  • mMontana
    mMontana Posts: 1,088
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    AFAIK they are, 1GBPS expressed in kbps. (1024x1024). Is also WAN interface setup the same way?

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