Hello Tung
You must remember that in such a topology, you have an overlay network and an underlay network. The underlay network in the lesson is the 192.168.1.0/24 network. This represents the Internet or any other network over which you will create your tunnels. This underlay network can be anything, using the same ISPs or different ISPs, just as long as the routers have connectivity between them. The routing between routers here is the responsibility of the ISPs. In fact, a static IP is needed only for the hub. The IP addresses of the spokes can actually be dynamic. Notice the hub configuration doesn’t include any information bout the IP addresses of the spokes.
Now the overlay network is the network you create that will be tunneled over the underlay network. In the lesson, this is the 172.16.1.0/24 network. Here, there is no actual routing protocol being used, but you can see, in the verification section of the lesson, that the spokes have been configured with a static default route to the hub, via the tunnel interface, which was “advertised” based on the flexVPN configuration. Similarly, the routes in the hub to the spokes were also advertised via IKEv2 and are based on the configuration in the IKEv2 authorization policy of the spokes themselves.
So in the lesson, the 0.0.0.0/0 is already configured in the spoke routers, due to the configuration implemented at the hub.
I hope this has been helpful!
Laz