WireGuard MTU Can Flap between 1420 and 1500
The default WireGuard MTU is typically 1420. However , I’ve observed cases where the wg interfaces will flap between 1500 and 1420. Explicitly configuring an MTU on the interface configuration fixes it
#1 Updated by Christian McDonald 3 months ago
I've nailed down clear reproduction steps...assuming that you have a WG tunnel and it's corresponding wg interface assigned.
- Go to the WG interface configuration and click save and apply.
- Check Status > Interfaces and the WG interface will have an MTU of 1500
- Go to the VPN > Wireguard > Tunnel configuration and click save
- Check Status > Interfaces again and the WG interface will now have an MTU of 1420
This is obviously problematic and was discovered by OSPF adjacency issues because the remote end had a fixed MTU of 1420, which is the WG default. I've disable MTU mismatch detection on both ends for the time being.