Setting MTU topic (again)

Quick question, I noticed that there are two places in LUCI to set the MTU, at the interfaces -> lan -> advanced and same for wan.
Looking at the output of the 'ip addr' command I see that there are still interfaces using the default 1500 mtu value, when having changed the values for LAN and WAN to 1492.
Should the MTU value not be the same for all interfaces (except local) when frames for instance pass on from wlan to lan to wan, so they don't get fragmented on the way?

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Ah something learned, thanks!
Reading up on PMTUD, it seems it relies heavily on ICMP working between the two end nodes. I noticed that the firewall only has rules for IP6 not IP4, should the router not be opened up for ICMP V4 for PMTUD to work properly?

  • Outgoing ICMP and ICMPv6 queries are allowed by default.
  • Incoming ICMP and ICMPv6 replies are allowed with connection tracking.
  • Incoming ICMP queries are not routed due to RFC 1918.
  • Incoming ICMPv6 queries are allowed with a firewall rule.