$ ping -c 5 192.168.33.1
PING 192.168.33.1 (192.168.33.1) 56(84) bytes of data.
--- 192.168.33.1 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4009ms

(at the same time in another console)
$ tcpdump host REMOTE
09:22:07.539582 IP LOCAL > REMOTE: ESP(spi=0xXXXXXX,seq=0x3), length 132
09:22:08.547608 IP LOCAL > REMOTE: ESP(spi=0xXXXXXX,seq=0x4), length 132
(... one packet per ping ...)