The overhead tax

The most tangible cost of PPPoE is the encapsulation itself. The PPPoE header consumes eight bytes, dropping the usable MTU from 1500 to 1492. That seemingly small number produces a long tail of operational pain: TCP MSS clamping on the BNG, path-MTU black holes, fragmentation, and the perennial support tickets that trace back to a handful of applications that don't handle a reduced MTU gracefully. IPoE carries no such tax. The subscriber gets a clean 1500-byte MTU, native to the Ethernet fabric, with no session wrapper to account for.