diff --git a/tools/testing/selftests/net/forwarding/README b/tools/testing/selftests/net/forwarding/README
index 4a0964c42860..b8a2af8fcfb7 100644
--- a/tools/testing/selftests/net/forwarding/README
+++ b/tools/testing/selftests/net/forwarding/README
@@ -46,6 +46,8 @@ Guidelines for Writing Tests
 
 o Where possible, reuse an existing topology for different tests instead
   of recreating the same topology.
+o Tests that use anything but the most trivial topologies should include
+  an ASCII art showing the topology.
 o Where possible, IPv6 and IPv4 addresses shall conform to RFC 3849 and
   RFC 5737, respectively.
 o Where possible, tests shall be written so that they can be reused by