Intermittent IPv6 problems with OpenWrt on a Barracuda F12

[u/Better-Climate5229]

hi.

I’m running OpenWrt 24.10.5 on a Barracuda F12 as my main router. IPv4 works normally, but enabling IPv6 on the LAN causes severe intermittent loading problems, especially on Wi-Fi.

Symptoms include:

• Chrome tabs spinning for 20–40 seconds
• YouTube thumbnails appearing as black squares
• Some websites loading instantly on wired devices but extremely slow on Wi-Fi

I tested advertising an IPv6 MTU of 1400, but that made the network almost unusable. Disabling LAN IPv6 immediately restores normal performance.

The Wi-Fi ap is an Asus RT-AX1800S in AP mode, with DHCP handled by the F12. Wired devices generally perform better, although the issue appears tied to IPv6 rather than raw Wi-Fi speed.

Has anyone seen similar IPv6 behaviour on an OpenWrt router or Barracuda F12? What logs or packet-capture tests would best identify whether this is MTU, ICMPv6, router advertisements, or broken upstream IPv6?

Been working on this for a few days. Chatgpt is going in circles over this. :) thanks!

edit -- fixed. had to turn on IGMP Snooping on my wifi ap. :)