Very slow speed over IPSec (Fritz!Box)

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Hi everyone,

I'm getting very slow speeds over a VPN set up between two routers (2x Fritz!Box 7390). The routers set up a VPN between them so I get the subnets 192.168.0.x and 192.168.1.x. I have set up appropriate NAT holes, even though for the VPN that shouldn't matter

I'm getting speeds in the 2 kb/s range, sometimes changes aren't even propagated to other clients.

As far as I can tell from the official documentation, the VPN uses IPSec/IKE with AES encryption. I have no real influence on any parameters of the VPN connection. All other applications work fine over VPN.

I've seen two similar threads (one with the exact same routers), but no replies.

Thanks and regards,


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Sorry for the delay. My MTU is 1500 anyway. Both connections are extremely stable VDSL2 connections with not troubles outside of this whatsoever.

I have disabled the VPN connection, disabled both Windows firewalls, to no avail. I double-checked the NAT port forwarding, tried out ĀµTorrent on both computers, using the same ports, which works fine.

Interestingly, after some messing around, I found out that the sync works one way - but not the other. That is, files from one network (machine called boreas) are being synched to the other (machine called atlas), but not the other way around. Both clients propagate changes, initiate transfers, but the transfers only work one way; the other way around, the transfer just sits there in the Transfers tab with 0.0kb/s speeds.

Third test: Got my sister to install on her computer (yet another network). Sync works fine between hers and mine, she can sync fine with the network containing atlas, but I get nothing from atlas.

In summary:

Three networks (atlas, boreas, other). Atlas & Boreas configured with NAT forwarding and disabled firewalls, other network as-is. Boreas->Atlas, Boreas->Other, Other->Boreas and Other->Atlas work fine. Atlas->Boreas doesn't.

I have sent all this (and the log files) in an email to

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