MTU/MRU
When the DSL modem sends and receives PPPoE-containing Ethernet frames across the Ethernet link to the router (or PPPoE-speaking single PC), there is an overhead of 8 bytes (2 for PPP, 6 for PPPoE) added within the payload of the Ethernet frame. This added overhead usually means that a reduced limit (so-called ‘MTU’ or ‘MRU’) of 1492 bytes is imposed on the length of IP packets sent or received, as opposed to the usual 1500 bytes for Ethernet networks. Some devices support RFC 4638, which allows a 1508 byte l Ethernet frame, sometimes called ‘baby jumbo frames’, thus allowing a full 1500 byte payload to be transported over a PPPoE session. This is advantageous because of the many companies who (incorrectly) block all ICMP traffic from exiting their network. This stops Path MTU discovery from working correctly and can cause problems for people accessing these networks with a MRU of less than 1500 bytes.
در اصل 1492 هست که با اون 8 تا میشه 1500 .