Hi all,

recently I upgraded from DSL to optical internet with 300/100 Mbps speed, and my ISP (A1 Croatia) provided me with Huawei EchoLife EG8145V5 ONT.
Basic internet works OK, but unfortunately ISP can't / won't set Huawei router to bridge mode, so I am stuck with double NAT, and DMZ, which works fine for the moment.
After that moment I wanted to upgrade / get rid of ISP's Huawei ONT, so I bought a Zyxel PMG3000-D20B GPON SFP module (software version V1.00(ABVJ.0)b3v), and stick it in my MikroTik hEX S. I set up S/N authentication on GPON and internet VLAN on MikroTik, and everything is working OK, but I have a problem with internet speed, on IPS's Huawei modem I get about 317/136 Mbps on Speedtest, but when using Zyxel GPON I get about 281/131 Mbps, so about 36 Mbps lower download speed then with ISP's Huawei modem.
I tried to measure and adjust MTU on Zyxel GPON using SSH, it was set to 1632, and I measured 1492 using ping, and set it to 1492, and then all hell breaks loose, nothing works, i set it back to 1632.
Now, I understand that this is not situation directly connected to a MikroTik device, but reather to Zyxel SFP GPON module, but is there anybody that have experience with Zyxel SFP GPON and MikroTik to tell me why I get lower internet speed, and how to fix it ?

First, is your connection using PPPoE? From the MTU value of 1492, it looks like that. If it's PPPoE, you can try to increase the MTU of the pppoe-out interface to 1500. I've a previous post about this topic in another thread:

viewtopic.php?t=207341#p1073496

2nd, do you have Fasttrack enabled on the hEX S. If it's not enabled, then the measured speed might be normal for the hEX. That's about near the limit for hEX/hEX S without fasttrack. Remember that now the router also has to perform PPPoE encapsulation/decapsulation, which uses a non-negligible amount of CPU power. So if previously it was near 100% limit at 300Mbps without having to handle PPPoE, a throughput of 280Mbps with PPPoE processing looks understandable.

But if you have fasttrack enabled and active, then the CPU won't be at the limit with those speed. It could be that previously the Huawei modem could use MTU 1500 with PPPoE and now your router doesn't. Try to increase the MTU like I wrote in the other post first.

2nd, do you have Fasttrack enabled on the hEX S. If it's not enabled, then the measured speed might be normal for the hEX.

Yes, that is it, thank You CGGXANNX !
The "problem" was Fasttrack, or better to say lack of it, I enabled Fasttrack, and now I get the full speed that ISP provides me.
btw, my connection don't use PPPoE, but DHCP.
Now, I get my "dream" setup, where optical cable is connected directly to my MikroTik, the simplest setup without any media converter, and bunch of ISP devices, and ISP can't control my devices.
Again, thank You very much !