Please see my post my isp supports RFC 4638 also l2mtu doesnt work on chr/x86 version but vyos/opnsense/ipfire all work on same setup with 1492 mtu except mikrotik why???lol..... the reason it drops to 1480 is because MikroTik sends a message out at the requested MTU and if it does not receive a response it will lower the MTU as that packet got silently discarded and therefore that MTU is not supported. It doesn't keep on sending 1 byte at a time to lower it. So often you can manually set it to 1492 to get more, it just reverts down to 1480 if anything larger is not working
1500 works fine, but you need a minimum of 1508 L2MTU (actually 1522 including ethernet) in the entire path between client and server. Many switches use 1518 by default, just enough room for a single VLAN + ethernet, and need to manually be increased. Once that is done hey presto 1500 byte MTU on PPPoE works fine
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Statistics: Posted by himurae — Fri Feb 28, 2025 10:37 am