Ping fails

I am using Mininet to start a topology with an  Hp SDN controller (version:2.7.18.0503) .

sudo mn --custom topo1.py --topo=mytopo --controller=remote,ip=<CONTROLLER_IP>

The messages shown after this command are :

*** Creating network
*** Adding controller
*** Adding hosts:
h1 h2 h3
*** Adding switches:
s1 s2 s3
*** Adding links:
(h1, s1) (h2, s2) (h3, s3) (s2, s1) (s3, s2)
*** Configuring hosts
h1 h2 h3
*** Starting controller
c0
*** Starting 3 switches
s1 s2 s3 ...
*** Starting CLI:
mininet>

Everything seems normal, but pingall fails :

mininet> pingall
*** Ping: testing ping reachability
h1 -> X 
h2 -> X 
*** Results: 100% dropped (0/2 received)