diff --git a/rtl/eth_mux.py b/rtl/eth_mux.py
index 6bd2421f7..d0d5cd22c 100755
--- a/rtl/eth_mux.py
+++ b/rtl/eth_mux.py
@@ -231,7 +231,7 @@ always @* begin
             // end of frame detection
             frame_next = ~current_input_tlast;
         end
-    end else if (enable & selected_input_eth_hdr_valid) begin
+    end else if (enable & ~output_eth_hdr_valid & selected_input_eth_hdr_valid) begin
         // start of frame, grab select value
         frame_next = 1;
         select_next = select;
diff --git a/rtl/eth_mux_4.v b/rtl/eth_mux_4.v
index 343411381..b7071462b 100644
--- a/rtl/eth_mux_4.v
+++ b/rtl/eth_mux_4.v
@@ -240,7 +240,7 @@ always @* begin
             // end of frame detection
             frame_next = ~current_input_tlast;
         end
-    end else if (enable & selected_input_eth_hdr_valid) begin
+    end else if (enable & ~output_eth_hdr_valid & selected_input_eth_hdr_valid) begin
         // start of frame, grab select value
         frame_next = 1;
         select_next = select;
diff --git a/rtl/eth_mux_64.py b/rtl/eth_mux_64.py
index 6630ad9df..4a3250d6a 100755
--- a/rtl/eth_mux_64.py
+++ b/rtl/eth_mux_64.py
@@ -236,7 +236,7 @@ always @* begin
             // end of frame detection
             frame_next = ~current_input_tlast;
         end
-    end else if (enable & selected_input_eth_hdr_valid) begin
+    end else if (enable & ~output_eth_hdr_valid & selected_input_eth_hdr_valid) begin
         // start of frame, grab select value
         frame_next = 1;
         select_next = select;
diff --git a/rtl/eth_mux_64_4.v b/rtl/eth_mux_64_4.v
index f8c904e28..cd80138e3 100644
--- a/rtl/eth_mux_64_4.v
+++ b/rtl/eth_mux_64_4.v
@@ -251,7 +251,7 @@ always @* begin
             // end of frame detection
             frame_next = ~current_input_tlast;
         end
-    end else if (enable & selected_input_eth_hdr_valid) begin
+    end else if (enable & ~output_eth_hdr_valid & selected_input_eth_hdr_valid) begin
         // start of frame, grab select value
         frame_next = 1;
         select_next = select;