pingall not working but ping works after Exercise 1

[Yamabuki-bakery]

Thanks for the exercises and tutorial!

I have followed the steps in exercise 1, using the provided VM and Docker containers, and finally made h1a and h1b can ping each other, by using h1a ping h1b and h1b ping h1a command.

But when I try to use mininet's pingall or pingallfull command, it seems that ping is 100% dropped:

I tried to debug this using tcpdump on h1a, and I can see the packets being sent and received when using h1a ping h1b and h1b ping h1a command.:

when I use pingall, tcpdump shows that no packet was sent or received:

I would like to use pingall to do a quick test. Since i'm new to mininet, I will very appreciate it if someone can help me locate the problem.