Setup MMS Clamping for PPPoE Fiber connection

I have a fiber connection over PPPoE. It is stable, no disconnects as I read about in other topics. However, I suspect the MTU is not set correctly.

My ISP specifies to use MSS Clamping with a value 1412 on the PPPoE interface. They ship a Fritzbox 5490 but they support using a different router and even reward that with a 2 euro discount on the monthly subscription fee. I use a Sitecom WLR-8100 running the SNAPSHOT (about a week old) ath79 firmware with Luci installed. This SNAPSHOT works on both v1.001 and v1.002 version.

The PPPoE connection is stable, no disconnects detected so far. However, sometimes a webpage or part of it loads later or not at all. I suspect issues to be MTU related.

OpenWRT support MSS Clamping in the Firewall settings. How does this actually work. And how to configure this correctly in Luci (or CLI if needed). How to configure the setting "MSS Clamping with value 1412"? I have asked my ISP if the number 1412 is a typo since 1452 makes more sense, but still, how can MSS Clamping be properly configured?

Any thoughs highly appreciated.

MSS clamping will force a smaller MTU on your LAN interface, so (buggy) clients do not think they can use 1500-byte packets.

If you activate it on the WAN interface (it's just a switch on the firewall configure took on luCi), then it's MTU will propagate to the other interfaces.

You need to set an mtu of 1412 bytes on WAN and enable mss clamping in addition - getting this wrong will indeed show itself in the problems you describe (not with all webpages, sadly). If you still have problems with that, you can try reducing the mtu even further (a too small mtu is inefficient and will ultimately affect your throughput, but it will work).

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Mmmh. MSS, or maximum segment size affects only TCP as far as I can tell, while a full MTU change will affect all transport protocols that use path MTU discovery. But that discovery is known not to work too well, mostly do to its reliance on ICMP messages passing unmolested/unfiltered from a MTU bottleneck back to the sender. Overly agressive firewalls broke the internet in that regard.