diff --git a/lib/librte_sched/rte_sched.c b/lib/librte_sched/rte_sched.c
index 7252f850d..5c88f1b62 100644
--- a/lib/librte_sched/rte_sched.c
+++ b/lib/librte_sched/rte_sched.c
@@ -216,7 +216,7 @@ struct rte_sched_port {
 	uint32_t n_pipes_per_subport;
 	uint32_t rate;
 	uint32_t mtu;
-	uint32_t frame_overhead;
+	int32_t frame_overhead;
 	uint16_t qsize[RTE_SCHED_TRAFFIC_CLASSES_PER_PIPE];
 	uint32_t n_pipe_profiles;
 	uint32_t pipe_tc3_rate_max;
@@ -643,7 +643,14 @@ rte_sched_port_config(struct rte_sched_port_params *params)
 	port->n_subports_per_port = params->n_subports_per_port;
 	port->n_pipes_per_subport = params->n_pipes_per_subport;
 	port->rate = params->rate;
-	port->mtu = params->mtu + params->frame_overhead;
+
+	/* Only add a +ve overhead.  A -ve overhead is to accommodate
+	 * for downstream TDM devices which won't have an ethernet header,
+	 * they obviously won't impact our local interface MTU size.
+	 */
+	port->mtu = params->mtu;
+	if (params->frame_overhead > 0)
+		port->mtu += params->frame_overhead;
 	port->frame_overhead = params->frame_overhead;
 	memcpy(port->qsize, params->qsize, sizeof(params->qsize));
 	port->n_pipe_profiles = params->n_pipe_profiles;
@@ -1613,13 +1620,21 @@ grinder_credits_check(struct rte_sched_port *port, uint32_t pos)
 	struct rte_sched_pipe *pipe = grinder->pipe;
 	struct rte_mbuf *pkt = grinder->pkt;
 	uint32_t tc_index = grinder->tc_index;
-	uint32_t pkt_len = pkt->pkt_len + port->frame_overhead;
+	uint32_t pkt_len;
+	int32_t tpkt_len;
 	uint32_t subport_tb_credits = subport->tb_credits;
 	uint32_t subport_tc_credits = subport->tc_credits[tc_index];
 	uint32_t pipe_tb_credits = pipe->tb_credits;
 	uint32_t pipe_tc_credits = pipe->tc_credits[tc_index];
 	int enough_credits;
 
+	/* Make sure we don't allow this to go -ve.  To accommodate
+	 * downstream TDM devices we may want to ignore the ethernet
+	 * header so allow -ve overhead values.
+	 */
+	tpkt_len = pkt->pkt_len + port->frame_overhead;
+	pkt_len = (tpkt_len < 0) ? 1 : tpkt_len;
+
 	/* Check queue credits */
 	enough_credits = (pkt_len <= subport_tb_credits) &&
 		(pkt_len <= subport_tc_credits) &&
@@ -1648,7 +1663,8 @@ grinder_credits_check(struct rte_sched_port *port, uint32_t pos)
 	struct rte_sched_pipe *pipe = grinder->pipe;
 	struct rte_mbuf *pkt = grinder->pkt;
 	uint32_t tc_index = grinder->tc_index;
-	uint32_t pkt_len = pkt->pkt_len + port->frame_overhead;
+	uint32_t pkt_len;
+	int32_t tpkt_len;
 	uint32_t subport_tb_credits = subport->tb_credits;
 	uint32_t subport_tc_credits = subport->tc_credits[tc_index];
 	uint32_t pipe_tb_credits = pipe->tb_credits;
@@ -1658,6 +1674,13 @@ grinder_credits_check(struct rte_sched_port *port, uint32_t pos)
 	uint32_t pipe_tc_ov_credits = pipe_tc_ov_mask1[tc_index];
 	int enough_credits;
 
+	/* Make sure we don't allow this to go -ve.  To accommodate
+	 * downstream TDM devices we may want to ignore the ethernet
+	 * header so allow -ve overhead values.
+	 */
+	tpkt_len = pkt->pkt_len + port->frame_overhead;
+	pkt_len = (tpkt_len < 0) ? 1 : tpkt_len;
+
 	/* Check pipe and subport credits */
 	enough_credits = (pkt_len <= subport_tb_credits) &&
 		(pkt_len <= subport_tc_credits) &&
@@ -1687,11 +1710,19 @@ grinder_schedule(struct rte_sched_port *port, uint32_t pos)
 	struct rte_sched_grinder *grinder = port->grinder + pos;
 	struct rte_sched_queue *queue = grinder->queue[grinder->qpos];
 	struct rte_mbuf *pkt = grinder->pkt;
-	uint32_t pkt_len = pkt->pkt_len + port->frame_overhead;
+	uint32_t pkt_len;
+	int32_t tpkt_len;
 
 	if (!grinder_credits_check(port, pos))
 		return 0;
 
+	/* Make sure we don't allow this to go -ve.  To accommodate
+	 * downstream TDM devices we may want to ignore the ethernet
+	 * header so allow -ve overhead values.
+	 */
+	tpkt_len = pkt->pkt_len + port->frame_overhead;
+	pkt_len = (tpkt_len < 0) ? 1 : tpkt_len;
+
 	/* Advance port time */
 	port->time += pkt_len;
 
diff --git a/lib/librte_sched/rte_sched.h b/lib/librte_sched/rte_sched.h
index e9c281726..63677eefc 100644
--- a/lib/librte_sched/rte_sched.h
+++ b/lib/librte_sched/rte_sched.h
@@ -219,7 +219,7 @@ struct rte_sched_port_params {
 	uint32_t mtu;                    /**< Maximum Ethernet frame size
 					  * (measured in bytes).
 					  * Should not include the framing overhead. */
-	uint32_t frame_overhead;         /**< Framing overhead per packet
+	int32_t frame_overhead;          /**< Framing overhead per packet
 					  * (measured in bytes) */
 	uint32_t n_subports_per_port;    /**< Number of subports */
 	uint32_t n_pipes_per_subport;    /**< Number of pipes per subport */
