i have mikrotik in x86 pc and recently i have created 6 vlan in my mikrotik. my main LAN have 1500 Max MTU and my every vlan also have MTU 1500 and all vlan have PPPoE Server in PPPoE Server MTU is 1480, But some of modem can not connect to pppoe server, i am confuse about this MTU is these MTU setting are correct for VLAN or not. please help
They are correct. PPPoE interface doesn’t equal VLAN interface, also not equal IP MTU.
You have different interfaces: PPPoE, Vlan, Bridge, MPLS and so on.
1500 MTU is IP MTU, and VLAN(ethernet) MTU is L2MTU. L2MTU by default is higher than IP MTU, because ethernet needs 14 bytes for ethernet header, with 802.1Q it is 4 bytes more, and with QinQ additional 4 bytes. PPPoE interface uses 1480 MTU, because of unknown nodes across the way. Classical PPPoE MTU is 1492 bytes(1492 IP MTU + 8 bytes PPPoE header = full 1500 IP MTU with TCP MSS 1480). In pure ethernet it will be 1514 bytes. Lower MTU we have, more chance there will no be fragmentation or traffic drops. 1480<1492.
After your ethernet frame terminates on ethernet interface(L2MTU), there will be PPPoE PADI frame from client which terminates on the server(1480 MTU). PPPoE server don’t manipulate L3 MTU, because it works on layer 2.
