diff --git a/gnpy/core/elements.py b/gnpy/core/elements.py
index 8255984ed..57c508f70 100644
--- a/gnpy/core/elements.py
+++ b/gnpy/core/elements.py
@@ -156,7 +156,8 @@ def update_snr(self, *args):
         # use raw_values so that the added SNR penalties are not cumulated
         snr_added = 0
         for s in args:
-            snr_added += db2lin(-s)
+            if s is not None:
+                snr_added += db2lin(-s)
         snr_added = -lin2db(snr_added)
         self.osnr_ase = snr_sum(self.raw_osnr_ase, self.baud_rate, snr_added)
         self.snr = snr_sum(self.raw_snr, self.baud_rate, snr_added)
diff --git a/gnpy/topology/request.py b/gnpy/topology/request.py
index 50ba90ee1..c69170668 100644
--- a/gnpy/topology/request.py
+++ b/gnpy/topology/request.py
@@ -338,17 +338,17 @@ def propagate(path, req, equipment):
         si = create_input_spectral_information(
             f_min=req.f_min, f_max=req.f_max, roll_off=req.roll_off, baud_rate=req.baud_rate,
             power=req.power, spacing=req.spacing, tx_osnr=req.tx_osnr, delta_pdb=req.offset_db)
+    roadm_osnr = []
     for i, el in enumerate(path):
         if isinstance(el, Roadm):
             si = el(si, degree=path[i + 1].uid, from_degree=path[i - 1].uid)
+            roadm_osnr.append(el.get_roadm_path(from_degree=path[i - 1].uid, to_degree=path[i + 1].uid).impairment.osnr)
         else:
             si = el(si)
     path[0].update_snr(si.tx_osnr)
     path[0].calc_penalties(req.penalties)
-    if any(isinstance(el, Roadm) for el in path):
-        path[-1].update_snr(si.tx_osnr, equipment['Roadm']['default'].add_drop_osnr)
-    else:
-        path[-1].update_snr(si.tx_osnr)
+    roadm_osnr.append(si.tx_osnr)
+    path[-1].update_snr(*roadm_osnr)
     path[-1].calc_penalties(req.penalties)
     return si
 
@@ -382,19 +382,21 @@ def propagate_and_optimize_mode(path, req, equipment):
                                                          roll_off=equipment['SI']['default'].roll_off,
                                                          baud_rate=this_br, power=req.power, spacing=req.spacing,
                                                          delta_pdb=this_offset, tx_osnr=req.tx_osnr)
+            roadm_osnr = []
             for i, el in enumerate(path):
                 if isinstance(el, Roadm):
                     spc_info = el(spc_info, degree=path[i + 1].uid, from_degree=path[i - 1].uid)
+                    roadm_osnr.append(el.get_roadm_path(from_degree=path[i - 1].uid, to_degree=path[i + 1].uid).impairment.osnr)
                 else:
                     spc_info = el(spc_info)
             for this_mode in modes_to_explore:
                 if path[-1].snr is not None:
                     path[0].update_snr(this_mode['tx_osnr'])
                     path[0].calc_penalties(this_mode['penalties'])
-                    if any(isinstance(el, Roadm) for el in path):
-                        path[-1].update_snr(this_mode['tx_osnr'], equipment['Roadm']['default'].add_drop_osnr)
-                    else:
-                        path[-1].update_snr(this_mode['tx_osnr'])
+                    roadm_osnr.append(this_mode['tx_osnr'])
+                    path[-1].update_snr(*roadm_osnr)
+                    # remove the tx_osnr from roadm_osnr list for the next iteration
+                    del roadm_osnr[-1]
                     path[-1].calc_penalties(this_mode['penalties'])
                     if round(min(path[-1].snr_01nm - path[-1].total_penalty), 2) \
                             > this_mode['OSNR'] + equipment['SI']['default'].sys_margins: