Lookup the manufactures data sheet a 21700 cell is just its external dimensions , you need to use the correct charging perimeters for your specific cells.
on the spec sheet you are looking for the following information
Standard charge CCCV, 2.5A, 4.20 V, 250mA cut-off
Maximum continuous charge CCCV, 6A, 4.20 V, 100mA cut-off
so a normal charge would be
"cc" constant current charge at 2.5 amps until the cell reaches 4.2 volts then switch to constant voltage charge mode
maintain 4.20 volts until the current drops to 250ma
you should also monitor the cell temperature
in my case the max charge temperature is 60c
so the charger should switch off before the cell gets to 60c
there are many charge control ic's available to manage this for you.
Do it properly or they will explode and set fire to anything around them