Collision strengths and collisional rate parameters are provided for transitions among the n=2 and 2p3l levels of C-like Fe xxi. The data have been computed in a 52-state Breit-Pauli approximation thus accounting for the most prominent relativistic effects. An R-matrix close-coupling calculation comprising partial wave contributions up to J=59/2 and of order 8000 energy points in the range up to 260 Ryd gave cross sections expected to be accurate to better than 20% for transitions among the n=2 levels. Detailed tables of the present data are available in electronic form at the CDS via anonymous ftp to 126.96.36.199 or via http://cdsweb.u-strasbg.fr/Abstract.html. Table 3 is only available in electronic form at the CDS.