We are getting an undefined reference `MPI_Exscan'. This function is used in src/mat/impls/nest/matnest.c. If I am not wrong, MPI_Exscan was introduced in MPI-2 but was not available in the previous standard. Apparently, the machine where we are compiling is very old and has MPI-1 only. I guess there should be a PETSC_HAVE_MPI_EXSCAN test in configure. Jose