Q-in-Q on a network from D-Link and Linux?

Question

linux

Good afternoon.
There is a diagram:
Computer---(2)D-Link DES-3028(1)---(2)D-Link DGS-3627G(1)---Server
Port numbers in parentheses are links :)
On the server has the eth0.12.34 interface raised.
Vlan 12 was created on DGS, assigned tagged to the port looking towards 3028 (2nd) and to the port looking towards the server (1st). The create vlan_translation ports 2 cvid 34 add svid 12 translation rule is configured. Port 1 is set to the nni role, port 2 is set to uni. All ports have tpid 0x8100 set.
Vlan 34 was created on DES, assigned tagged to the port, looking towards the DGS and tagged to the computer port.
On the network interface of the computer and on the eth0.12.34 interface, ips from the same subnet are assigned, pings go, everything seems to work.
However, the traffic outgoing from the computer runs with some obvious problems. If you measure the speed using iperf, then in the direction from the server to the computer everything is in order, and in the opposite direction - 10-20 kilobits. If you use a service like speedtest.net (the server just routes traffic to the Internet, NAT is not used), then the download test passes fine, but the upload test starts with a long delay and shows low values. Similar problems are observed with any outgoing traffic, when using different software, different computers, different DES and even different DGS.
When using vlan with one tag, everything is fine. At the same time, we create vlan 34 on the DGS, assign ports tagged to 1.2 and create another translation rule: create vlan_translation ports 1 cvid 34 replace svid 34. On the server, we use vlan 34. In this case, there are no problems with traffic in any direction.
Intuition suggests a problem with MTU. Reducing the MTU on the computer to 1492 seems to solve the problem.
Question: how can I solve the problem without changing the MTU on the computer?

Answer 1

The issue was resolved by using the following rule on the server:
iptables -t mangle -A POSTROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1454

Answer 2

No way, why do you need a vlan in a vlan, 1024 vlans are not enough?