BUGFIX: Avoid marking single-provider links as multi-provider

Fixes #1658

netsim/providers/__init__.py

@@ -236,17 +236,21 @@ def pre_transform(self,topology : Box) -> None:
       return
 
     for l in topology.links:
+      providers = {}
       for intf in l.interfaces:
         node = topology.nodes[intf.node]
-        if not 'provider' in node:
-          continue
-
-        p_name = topology.provider                          # Get primary and secondary provider
-        s_name = node.provider                              # ... to make the rest of the code more readable
-
-        l[p_name].provider[s_name] = True                   # Collect secondary link provider(s)
-        if 'uplink' in l[p_name]:                           # ... and copy primary uplink to secondary uplink
-          l[s_name].uplink = l[p_name].uplink
+        provider = node.get('provider',topology.provider)
+        providers[ provider ] = True
+
+      if len(providers)<=1:
+        continue                                          # Not a multi-provider link - continue
+
+      p_name = topology.provider                          # Get primary and secondary provider
+      s_name = node.provider                              # ... to make the rest of the code more readable
+
+      l[p_name].provider[s_name] = True                   # Collect secondary link provider(s)
+      if 'uplink' in l[p_name]:                           # ... and copy primary uplink to secondary uplink
+        l[s_name].uplink = l[p_name].uplink
 
   """
   Generic provider pre-output transform: remove loopback links