Setting MTU on VDSL2 UK

Hi all,

Hope you're well. Got my HH5A today, trying to set up the network. In the network -> interfaces -> devices section, I've forgot the default settings. I accidentally changed something, knocked the internet offline, and now I'm not sure whats correct.

I'm on BT Broadband in the UK. Looking at this link:

It says config interface 'wan' option MTU should be 1500.

My settings are:
pppoe-wan 1492
dsl0.101 1500
eth0 1508
wan 1500.

I'm confused as to which I should be setting as 1500. Am I right in that it should be in the interfaces tab, Click Edit on this one:
Advanced settings, override MTU from 1492 to 1500?

Update 05/02 - If I change the pppoe-wan ipv4 MTU to 1500 it works fine still (i think) and I can ping 1472 instead of 1464. Assuming this is the only change I need to do.

Do I change ipv6 as well?

Instead of overriding I believe I previously configured the pppoe-wan to 1500, I think it worked.
Currently at these settings I can only ping a packet of 1464.
When it was overridden or set to 1500 I could ping 1472.

Is eth0 as 1508 correct - I'm reading stuff about an 8 packet overhead which I guess is automatically in the right place..

Normally, the mtu with PPPoE should be 1492 - some ISPs allow baby-jumbo frames, to let you pass 1500 bytes net mtu through it, but those are relatively rare (so it's unclear if this applies to you).

BT supports baby jumbo frames, so:

  1. Delete all mtu settings from Network > Interfaces > Devices which should hopefully revert them to their defaults.

  2. In /etc/config/network for interface wan set: option mtu '1508'

  3. Optionally in the same section set: option pppd_options 'debug' which should display PPP-max-payload in the syslog which means it is (attempting to) use baby jumbo frames.

  4. Test ping with 1472

Maybe I'm being really dumb but what does /etc/config/network mean, how do I get there?

So after some testing, 1500 MTU on pppoe-wan does let me send 1472 packets instead of 1464. It seems to be setting the ipv6 to 1500 as well.

So on this website: I get a 10/10 score on both MTUs 1492 and 1500. There's a tab called other IPv6 sites. When I'm set to 1492 I can access all but 2 just fine. When I'm set to 1500 I can access only a couple. Seems IPv6 MTU of 1500 is breaking my access to websites and I don't understand why.

BT should support baby jumbo frames, and appears to, so I'm not sure why this is happening. What if I set a different MTU size for IPv4 and 6, will this break things further?

Not in the UK, I believe that the common operator of DSL lines supports baby-jumbo frames and so this is generally available in the UK...