fixing system test issue where non-existent interface could be selected

test/lib/systestlib.py

@@ -66,9 +66,15 @@ def sort_dict_by_keys(self, d):
 
 
 def random_interface(dut, exclude=None):
+    # interfaces read in 'show run all' and those actually present may differ,
+    # thus interface list must be picked from the actually present
+    if not getattr( random_interface, 'present', False ):
+        random_interface.present = dut.run_commands(
+            'show interfaces', send_enable=False )[ 0 ][ 'interfaces' ].keys()
     exclude = [] if exclude is None else exclude
     interfaces = dut.api('interfaces')
-    names = [name for name in list(interfaces.keys()) if name.startswith('Et')]
+    names = [ name for name in list(interfaces.keys()) if name.startswith('Et') ]
+    names = [ name for name in names if name in random_interface.present ]
 
     exclude_interfaces = dut.settings.get('exclude_interfaces', [])
     if exclude_interfaces: