I will give an example of increasing MTU on Huawei MA5600 OLT.
Jumbo frame is enabled on UPLINK boards, for example H801X2CS, H801GICF, H801MCUD1, etc., I note that the default MTU is 2052 bytes, if you enable Jumbo frame it will be 9 kilobytes.
On Huawei ONTs, the default MTU is also 2000 bytes, I checked on hg8310m and hg8010, on many third-party ONTs, the default MTU is 1500 bytes and the possibility of increasing is rare.
To enable jumbo frame, connect to OLT and switch to configuration mode:
Then select the required UPLINK board and enable jumbo frame on both ports:
interface giu 0/9 jumbo-frame 1 enable jumbo-frame 0 enable
On the test, I turned on the jumbo frame on the Huawei MA5683T with 5000 online clients, the enabling did not affect the operation of the Internet, there were no interruptions.
I remembered the old D-Link DGS-3100 switch, which needed to be rebooted, because after turning on the jumbo frame on it, the traffic stopped flowing from the clients.