diff --git a/exercise4/wave_2d_parallel.c b/exercise4/wave_2d_parallel.c
index ba49f2f..0b14a81 100644
--- a/exercise4/wave_2d_parallel.c
+++ b/exercise4/wave_2d_parallel.c
@@ -46,6 +46,8 @@ MPI_Comm cartesian_comm;
 int coordinates[2], dims[2] = { 0 };
 int cartesian_rank;
 
+int up, down, left, right;
+int up_left, up_right, down_left, down_right;
 // END: T1b
 
 // Simulation parameters: size, step count, and how often to save the state
@@ -231,7 +233,49 @@ void get_corner_procs(void) {
 // Communicate the border between processes.
 void border_exchange(void) {
   // BEGIN: T6
-  ;
+
+  // rows
+  MPI_Sendrecv(&U(0, 0), local_N, MPI_DOUBLE, up, 0,
+               &U(local_M, 0), local_N, MPI_DOUBLE, down, 0,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(local_M - 1, 0), local_N, MPI_DOUBLE, down, 1,
+               &U(-1, 0), local_N, MPI_DOUBLE, up, 1,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  // columns
+  MPI_Datatype column_type;
+  MPI_Type_vector(local_M, 1, local_N + 2, MPI_DOUBLE, &column_type);
+  MPI_Type_commit(&column_type);
+
+  MPI_Sendrecv(&U(0, 0), 1, column_type, left, 2,
+               &U(0, local_N), 1, column_type, right, 2,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(0, local_N - 1), 1, column_type, right, 3,
+               &U(0, -1), 1, column_type, left, 3,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Type_free(&column_type);
+
+  // corners
+  get_corner_procs();
+
+  MPI_Sendrecv(&U(0, 0), 1, MPI_DOUBLE, up_left, 4,
+               &U(local_M, local_N), 1, MPI_DOUBLE, down_right, 4,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(0, local_N - 1), 1, MPI_DOUBLE, up_right, 5,
+               &U(local_M, -1), 1, MPI_DOUBLE, down_left, 5,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(local_M - 1, 0), 1, MPI_DOUBLE, down_left, 6,
+               &U(-1, local_N), 1, MPI_DOUBLE, up_right, 6,
+               cartesian_comm, MPI_STATUS_IGNORE);
+
+  MPI_Sendrecv(&U(local_M - 1, local_N - 1), 1, MPI_DOUBLE, down_right, 7,
+               &U(-1, -1), 1, MPI_DOUBLE, up_left, 7,
+               cartesian_comm, MPI_STATUS_IGNORE);
   // END: T6
 }