diff --git a/tools/testing/selftests/net/forwarding/forwarding.config.sample b/tools/testing/selftests/net/forwarding/forwarding.config.sample
index 1fc4f0242fc5..f1ca95e79a65 100644
--- a/tools/testing/selftests/net/forwarding/forwarding.config.sample
+++ b/tools/testing/selftests/net/forwarding/forwarding.config.sample
@@ -3,51 +3,28 @@
 
 ##############################################################################
 # Topology description. p1 looped back to p2, p3 to p4 and so on.
-declare -A NETIFS
 
-NETIFS[p1]=veth0
-NETIFS[p2]=veth1
-NETIFS[p3]=veth2
-NETIFS[p4]=veth3
-NETIFS[p5]=veth4
-NETIFS[p6]=veth5
-NETIFS[p7]=veth6
-NETIFS[p8]=veth7
-NETIFS[p9]=veth8
-NETIFS[p10]=veth9
+NETIFS=(
+	[p1]=veth0
+	[p2]=veth1
+	[p3]=veth2
+	[p4]=veth3
+	[p5]=veth4
+	[p6]=veth5
+	[p7]=veth6
+	[p8]=veth7
+	[p9]=veth8
+	[p10]=veth9
+)
 
 # Port that does not have a cable connected.
 NETIF_NO_CABLE=eth8
 
 ##############################################################################
-# Defines
+# In addition to the topology-related variables, it is also possible to override
+# in this file other variables that net/lib.sh, net/forwarding/lib.sh or other
+# libraries or selftests use. E.g.:
 
-# IPv4 ping utility name
-PING=ping
-# IPv6 ping utility name. Some distributions use 'ping' for IPv6.
 PING6=ping6
-# Packet generator. Some distributions use 'mz'.
 MZ=mausezahn
-# mausezahn delay between transmissions in microseconds.
-MZ_DELAY=0
-# Time to wait after interfaces participating in the test are all UP
 WAIT_TIME=5
-# Whether to pause on failure or not.
-PAUSE_ON_FAIL=no
-# Whether to pause on cleanup or not.
-PAUSE_ON_CLEANUP=no
-# Type of network interface to create
-NETIF_TYPE=veth
-# Whether to create virtual interfaces (veth) or not
-NETIF_CREATE=yes
-# Timeout (in seconds) before ping exits regardless of how many packets have
-# been sent or received
-PING_TIMEOUT=5
-# Minimum ageing_time (in centiseconds) supported by hardware
-LOW_AGEING_TIME=1000
-# Flag for tc match, supposed to be skip_sw/skip_hw which means do not process
-# filter by software/hardware
-TC_FLAG=skip_hw
-# IPv6 traceroute utility name.
-TROUTE6=traceroute6
-
diff --git a/tools/testing/selftests/net/forwarding/lib.sh b/tools/testing/selftests/net/forwarding/lib.sh
index ee48c4603ff0..dbd4348f85b8 100644
--- a/tools/testing/selftests/net/forwarding/lib.sh
+++ b/tools/testing/selftests/net/forwarding/lib.sh
@@ -1,34 +1,83 @@
 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+##############################################################################
+# Topology description. p1 looped back to p2, p3 to p4 and so on.
+
+declare -A NETIFS=(
+    [p1]=veth0
+    [p2]=veth1
+    [p3]=veth2
+    [p4]=veth3
+    [p5]=veth4
+    [p6]=veth5
+    [p7]=veth6
+    [p8]=veth7
+    [p9]=veth8
+    [p10]=veth9
+)
+
+# Port that does not have a cable connected.
+: "${NETIF_NO_CABLE:=eth8}"
+
 ##############################################################################
 # Defines
 
-# Can be overridden by the configuration file.
+# Networking utilities.
 : "${PING:=ping}"
-: "${PING6:=ping6}"
-: "${MZ:=mausezahn}"
-: "${MZ_DELAY:=0}"
+: "${PING6:=ping6}"	# Some distros just use ping.
 : "${ARPING:=arping}"
+: "${TROUTE6:=traceroute6}"
+
+# Packet generator.
+: "${MZ:=mausezahn}"	# Some distributions use 'mz'.
+: "${MZ_DELAY:=0}"
+
+# Host configuration tools.
 : "${TEAMD:=teamd}"
-: "${WAIT_TIME:=5}"
-: "${PAUSE_ON_FAIL:=no}"
-: "${PAUSE_ON_CLEANUP:=no}"
-: "${NETIF_TYPE:=veth}"
-: "${NETIF_CREATE:=yes}"
 : "${MCD:=smcrouted}"
 : "${MC_CLI:=smcroutectl}"
-: "${PING_COUNT:=10}"
-: "${PING_TIMEOUT:=5}"
-: "${WAIT_TIMEOUT:=20}"
+
+# Constants for netdevice bring-up:
+# Default time in seconds to wait for an interface to come up before giving up
+# and bailing out. Used during initial setup.
 : "${INTERFACE_TIMEOUT:=600}"
+# Like INTERFACE_TIMEOUT, but default for ad-hoc waiting in testing scripts.
+: "${WAIT_TIMEOUT:=20}"
+# Time to wait after interfaces participating in the test are all UP.
+: "${WAIT_TIME:=5}"
+
+# Whether to pause on, respectively, after a failure and before cleanup.
+: "${PAUSE_ON_FAIL:=no}"
+: "${PAUSE_ON_CLEANUP:=no}"
+
+# Whether to create virtual interfaces, and what netdevice type they should be.
+: "${NETIF_CREATE:=yes}"
+: "${NETIF_TYPE:=veth}"
+
+# Constants for ping tests:
+# How many packets should be sent.
+: "${PING_COUNT:=10}"
+# Timeout (in seconds) before ping exits regardless of how many packets have
+# been sent or received
+: "${PING_TIMEOUT:=5}"
+
+# Minimum ageing_time (in centiseconds) supported by hardware
 : "${LOW_AGEING_TIME:=1000}"
+
+# Whether to check for availability of certain tools.
 : "${REQUIRE_JQ:=yes}"
 : "${REQUIRE_MZ:=yes}"
 : "${REQUIRE_MTOOLS:=no}"
+
+# Whether to override MAC addresses on interfaces participating in the test.
 : "${STABLE_MAC_ADDRS:=no}"
+
+# Flags for tcpdump
 : "${TCPDUMP_EXTRA_FLAGS:=}"
-: "${TROUTE6:=traceroute6}"
+
+# Flags for TC filters.
+: "${TC_FLAG:=skip_hw}"
 
 net_forwarding_dir=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")