For ME60, version V6R2 support this tag, its rule is as below.
If CPE does not send PPP LCP MRU actively, final online subscriber MTU will be the minimum(1492, VT MTU, Interface MTU-8).
If CPE sends PPP LCP MRU actively,
1. When a "PPP-Max-Payload" tag is confirmed in PPPoE Discovery phase,
MTU will use the minimum(PPP-Max-Payload-Tag, CPE MRU, VT MTU, Interface MTU-8).
2. When a "PPP-Max-Payload" tag is not received in PPPoE Discovery phase,
MTU will use the minimum (CPE MRU, VT MTU, Interface MTU-8).
Why need Interface MTU minus 8bytes? Because PPPoE link, PPP MTU need to exclude PPPoE header 6 bytes and PPP protocol type ID 2bytes.
So according to the rules above, if the CPE does not send MRU actively, then it have to send PPP-max-payload in PADI and PADR, otherwize, no matter ME60 MTU change to any big value, online subscriber MTU can only reach 1492.
For example, if CPE send MRU 1600 actively, and ME60 VT MTU is 1500, then online user MTU will be 1500, if ME60 VT MTU is 1620, then online user MTU will be 1600.