+----------+                       +------------+
lan a.b.c.0/24 -- | VPN GW A |=== ipsec tunnel 1 === |  VPN GW Z  |-- lan 192.168.0.0/24
+ a.b.c.199/32    |IP a.b.c.d|=== ipsec tunnel 2 === | IP w.x.y.z |       192.168.1.0/24
                  +----------+                       +------------+       + a.b.c.199/32

GW A is proxy arping a.b.c.199 so it can tunnel all traffic for it over to GW Z, so that
a.b.c.199 IP appears on Z's local LAN.