Help calculate MTU, ISP's 1448

[Roman Mamedov]

On Tue, 26 Feb 2019 12:39:50 +0000
"STR ." <strykar at hotmail.com> wrote:

> I have Fiber to our apartment complex basement, from there Cat6 runs to
> each apartment. The ISP/apartment service provider suggests an MTU of
> 1448, which I set for the PPPoE interface on my OpenWRT router.

It could be that your ISP meant this as the pre-PPPoE MTU, so 1448 on ethX
link, but 1440 inside PPPoE.

> I read 
> https://lists.zx2c4.com/pipermail/wireguard/2017-December/002201.html
> which comes to (assuming 1500 byte MTU) to 60 bytes (IPv6) to 80 bytes less to account for Wireguard protocol overhead.
> 
> Using this info, I tried an MTU of both (1448-80=1368) and (1448-
> 60=1388).
> As my ISP assigns only IPv4, I expected an MTU of 1388 to work, which I
> set on the Wireguard interface in OpenWRT.
> 
> However, when set to 1388, almost everything works except any Google
> related sites like Maps, Gmail, YT etc.
> When set to 1368, everything works and it's the way I have it setup
> right now.

Try 1380 for a bit.

> What am I missing here?
> Why won't Google sites load via my WG VPN when the MTU is set to 1388?
> 
> If it helps, I host the WG server on Google's cloud platform and was
> informed that GCP has an MTU of 1460 bytes.

Are you setting the same (lower of the two maximums) MTU on both ends of the
wireguard tunnel? (You should)

-- 
With respect,
Roman