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+# What's `runner.py` doing?
+
+## Common configurations
+
+### VPP1
+```
+create host-interface name eth1
+set int ip addr host-eth1 A1::1/120
+set int state host-eth1 up
+ip route add ::/0 via host-eth1 A1::2
+```
+
+
+### VPP2
+
+```
+create host-interface name eth1
+set int ip addr host-eth1 A1::2/120
+create host-interface name eth2
+set int ip addr host-eth2 A2::1/120
+set int state host-eth1 up
+set int state host-eth2 up
+ip route add ::/0 via host-eth2 A2::2
+```
+
+
+### VPP3
+
+```
+create host-interface name eth1
+set int ip addr host-eth1 A2::2/120
+create host-interface name eth2
+set int ip addr host-eth2 A3::1/120
+set int state host-eth1 up
+set int state host-eth2 up
+ip route add ::/0 via host-eth1 A2::1
+```
+
+### VPP4
+
+```
+create host-interface name eth1
+set int ip addr host-eth1 A3::2/120
+set int state host-eth1 up
+ip route add ::/0 via host-eth1 A3::1
+```
+
+
+## Drop-in for GTP-U over IPv4
+
+Drop-in mode is handy to test both GTP-U-to-SRv6 and SRv6-to-GTP-U functions at same time. Let's see what's happened when you run `test gtp4`:
+
+    $ ./runner.py test gtp4
+
+
+Setting up a virtual interface of packet generator:
+
+#### VPP1
+
+```
+create packet-generator interface pg0
+set int mac address pg0 aa:bb:cc:dd:ee:01
+set int ip addr pg0 172.16.0.1/30
+set ip arp pg0 172.16.0.2/30 aa:bb:cc:dd:ee:02
+```
+
+#### VPP4
+
+```
+create packet-generator interface pg0
+set int mac address pg0 aa:bb:cc:dd:ee:11
+set int ip addr pg0 1.0.0.2/30
+set ip arp pg0 1.0.0.1 aa:bb:cc:dd:ee:22
+```
+
+SRv6 and IP routing settings:
+
+#### VPP1
+
+```
+sr policy add bsid D4:: next D2:: next D3::
+sr policy add bsid D5:: behavior t.m.gtp4.d D4::/32 v6src_prefix C1::/64 nhtype ipv4
+sr steer l3 172.20.0.1/32 via bsid D5::
+```
+
+#### VPP2
+
+```
+sr localsid address D2:: behavior end
+ip route add D3::/128 via host-eth2 A2::2
+```
+
+#### VPP3
+
+```
+sr localsid address D3:: behavior end
+ip route add D4::/32 via host-eth2 A3::2
+```
+
+#### VPP4
+
+```
+sr localsid prefix D4::/32 behavior end.m.gtp4.e v4src_position 64
+ip route add 172.20.0.1/32 via pg0 1.0.0.1
+```