How to fix packet loss on MikroTik RB951G-2HnD via PPPoE connection?

Question

Mikrotik

Mikrotik RB951G-2HnD has the latest RouterOS 6.19 installed.
The provider (Rostelecom) is connected to the first port of the router, the connection speed in Mikrotik is defined as 100 full duplex.
When raising a PPPoE connection with the provider, about 40% of packets are lost through this connection. I tried ping 8.8.8.8 and ping ya.ru and just opened sites via http.
It is important that when using old equipment (server on Intel Atom), such losses are not detected. I tried to connect a laptop with Ubuntu directly, too, everything works without problems.
What I did:
Changed MTU PPPoE connections and flow control Ethernet port1.
I changed the router to exclude the non-operability of iron.
Changed the physical port of the router to the 5th effect was preserved.
On the forum, Mikrotik found a similar problem, which is supposedly fixed in version 6.11 of RouterOS.
Also, as suggested on the forum, Mikrotik turned on between Mikrotik and the Dlink-DGS 1005d provider and the packet loss disappeared.
How to fix the packet loss problem? I want to connect directly, not through add. equipment.

Answer 1

You need to change not MTU, but MSS
chain=forward action=change-mss new-mss=1452 tcp-flags=syn,!rst in-interface=all-ppp protocol=tcp tcp-mss=1453-65535
chain=forward action=change-mss new-mss=1452 tcp-flags=syn,!rst out-interface=all-ppp protocol=tcp tcp-mss=1453-65535
wiki.mikrotik.com/wiki/Manual:IP/Firewall/Mangle

Answer 2

The connection speed in Mikrotik is defined as 100 full duplex.

Please specify, are the speed and duplex determined automatically or are they hard-coded? Are there any CRC errors (sometimes the term FCS is used) on incoming frames on the port connected to the ISP? What mode does the laptop interface work in when connected (100 Mbps or 1 Gbps)? If there are CRC errors on the Mikrotik port and the laptop port is running at 1 Gb / s, then try setting the Mikrotik port to 1 Gb / s.

Answer 3

The machine itself will set 1452 if the mtu is 1492. It's
banal to go into the mangle and make a print. Before installing new chains)