incorrect MTU upon WAN(pppoe) interface reload from INTERFACES: OVERVIEW

[ryp43]

Hi All!

I have set MTU on the WAN interface to 1460 (Calculated PPP MTU: 1452).

Upon router reboot the MTU has the correct value:

sudo ifconfig
pppoe0: flags=88d1<UP,POINTOPOINT,RUNNING,NOARP,SIMPLEX,MULTICAST> metric 0 mtu 1452
        description: WAN

But if I initiate "reload" from INTERFACES: OVERVIEW the MTU resets to 1460

sudo ifconfig
pppoe0: flags=88d1<UP,POINTOPOINT,RUNNING,NOARP,SIMPLEX,MULTICAST> metric 0 mtu 1460

Can it be a defect?

Thanks in advance!

[meyergru]

I just tried and any MTU that was larger than the physical interface cannot accomodate is overruled.

Your hardware probably has an MTU of 1460 and there is a VLAN oder PPPoE around it (or a VPN tunnel).

[ryp43]

Thanks!

I'm not aware of any hardware MTU limiting.

My setup is a VDSL2 router in bridge mode -> OPNsense.

[meyergru]

As I expected. Ethernet MTU is 1500 or 1460 bytes and PPP takes another 8 bytes, so the calculated MTU of 1452 seems about right.

Same thing here, except that my physical ethernet interface can do >=1500 bytes minus PPP, so my calculated value is 1492.