PPPoE MTU as specified by ISP not working

RouterOS General
1

My ISP specifies the settings for a custom router as below:

2021-05-16_19-46.png
However, when I set an MTU of 1508 to my eth1 and vlan6, and MTU & MRU & MMRU of 1500 to my PPPoE, it can’t connect.

I did get it working by setting my eth1 MTU to 1500, vlan6 MTU to 1508, and unset any MTU on my PPPoE, resulting in the setup below:
RouterOS-interfaces.png
I’m not sure why the settings as specified by my ISP aren’t working like they should, they do work like that on an Edgerouter with OpenWrt.
I also have no clue about the Layer 2 MTU values, or what they should be, so I haven’t touched them.

Any ideas how I can fix this?

2

ether1 MTU should be 1512 (1508 + the VLAN Tag of 4 byte).
vlan6 MTU should be 1508.
PPPoE MTU/MRU 1500, you don’t touch MRRU.

LE: for auto MTU/MRU to work properly on the PPPoE client, you have to set ether1 MTU to 1524 and vlan6 MTU to 1520; yes, 12 bytes more for each, because.. MikroTik.

3

Thanks for your reply, finally got around to testing it.

With both options, the PPPoE MTU starts at 1500 when a connection is established, and then a couple seconds later it drops down to 1480. Not the end of the world, but 1500 would’ve been nice. Any idea why that might be?

4

If that is the case, something between ether1 and your ISP doesn’t support Baby Jumbo Frames.
I had the same issue on RB4011 using the SFP port, which required a disable/enable of the SFP port after reboot for the MTU setting to get applied. Think that was fixed in the latest beta.
What device is it? try a disable/enable of ether1.
I’m still having this issue with a RB3011 but I suspect there might be a switch that I didn’t know about or the ISP’s ONT is faulty. I can’t verify since I can’t go to that location for the time beeing.
I also have other RB3011 working fine with Baby Jumbo Frames so I don’t think it’s a RB issue this time.

5

Ahhh I see. I did, indeed move to the SFP port of my 4011. I’ll try to disable/enable, and otherwise upgrade to beta.

6

disable/enable worked :D. Suprised such a small thing can resolve this

7

Hello there,

one short question, why 12 bytes in particular? And this should be set for the normal “MTU“ option, not for L2 MTU right? Thanks in advance!

8

PPPoE adds 8 bytes of overhead. VLAN adds 4 bytes of overhead. Together 12 bytes in total.

No, both are about L2 MTU, L3 MTU only applies when there's IP address on the interface to the traffic targeting that IP (can be indirectly as in case of a router but it involves IP/IPv6 layer anyway), in case of PPPoE over VLAN it's the pppoe-out interface which is involved with L3 traffic, VLAN interface and physical are not.