Fix radvd warning invalid MTU 1500 on MikroTik

After upgrading to the latest stable RouterOS, I suddenly got many of the following log entries: Sep 9 06:09:43 fw-router-1-wan.example.lan radvd,warning invalid mtu 1500 on vpn_tunnel_45 from fe80::1111:2222:3333 Sep 9 06:11:09 fw-router-1-wan.example.lan radvd,warning invalid mtu 1500 on vpn_tunnel_50 from fe80::4444:5555:6666:7777 Sep 9 06:14:26 fw-router-1-wan.example.lan radvd,warning invalid mtu 1500 on vpn_tunnel_51 from fe80::8888:9999:aaaa:bbbb Sep 9 06:15:23 fw-router-1-wan.example.lan radvd,warning invalid mtu 1500 on vpn_tunnel_45 from fe80::1111:2222:3333 Turns out that for some reason (maybe some old manual config?), the following was configured on the devices: /ipv6 nd set [ find default=yes ] advertise-dns=yes hop-limit=64 mtu=1500 Thus to fix it, we need to set the mtu parameter to unspecified (so that MTU values are not added to the neighbor discovery messages and radvd stops complaining about the MTU missmatch): /ipv6 nd set [ find default=yes ] mtu=unspecified…