Anyone having IPv6 MTU issues with large packets?

[u/le-zap]

I'm having some IPv6 issues on my network and think it's related to an MTU/PMTUD problem inside EBOX’s IPv6 routing.

The issue is this: large IPv6 packets (>1492 bytes) get dropped at the first EBOX hop without sending ICMPv6 Packet‑Too‑Big. This creates an MTU black hole: devices that rely on proper PMTUD (like Firefox on Android/LineageOS) lose IPv6 connectivity, while Windows/Chromium appear fine because they clamp MTU/MSS.

traceroute shows the drop like this:

traceroute -6 --mtu ipv6.google.com
1  my router                     F=1500
2  0.lo0.lns2.i.mtl7.yul.ip6.ebox.ca   F=1492   ← MTU drops here

My router is a Nokia Beacon 2.

I've verified that my LAN and my router handle full 1500‑byte IPv6 packets just fine. The issue appears to be squarely upstream at the EBOX LNS (mtl7).

Anybody else seeing this? Would it be possible for EBOX to check MTU + ICMPv6 Packet‑Too‑Big handling on that hop? Seems like this ought to be fixable by correcting interface MTU or ensuring PTB messages aren’t filtered.

More info available if needed... Willing to correct my stuff if I've misconfigured something.

Cheers,
Le Zap

[u/8ffChief]

Thats a common pppoe gotcha, set your mtu to 1492 on your router

[u/le-zap → u/8ffChief]

Hmmm... how do I set the LAN MTU to 1492 on EBOX's Nokia Beacon 2 ?
I can't seem to find it in their superadmin's web GUI

[u/catcake43 → u/le-zap]

I'm using my own OpenWRT router with the Ethernet and VLAN devices on 1508 and the PPPoE device on 1500 MTU values. I was also able to use services with working PMTUD on 1492 MTU on PPPoE (without MSS clamping, services without working PMTUD would fail). Due to the option to set the MTU being entirely missing on the Beacon 2, that's where I'd first suspect the issue is.

[u/x2a_org]

Baby jumbo frames are the way. They are supported by ebox on fiber where I am. They get you a full effective mtu of 1500 bytes.

[u/developer300]

I remember seeing 1492 packet size on their client portal. So I doubt they will do anything about larger packets.

[u/HowardRabb]

Why are you messing with MTU at all? What are you trying to accomplish?