Fix #520

mininet/node.py

@@ -1143,7 +1143,7 @@ class OVSSwitch( Switch ):
         if self.protocols and not self.isOldOVS():
             opts += ' protocols=%s' % self.protocols
         if self.stp and self.failMode == 'standalone':
-            opts += ' stp_enable=true' % self
+            opts += ' stp_enable=true'
         return opts
 
     def start( self, controllers ):