How does a LPN join mesh network after reboot it?

Dear Squad Team,

I'm newbie Zephyr developer.

I make a mesh network with sample code of Zephyr (mesh_provisioner, mesh):

1. 1 Mesh_provisioner node, features: Relay, Friend (samples/mesh_provisioner)

2. Low Power Nodes (samples/mesh)

Step 1: Provision LPN node: ok

Step 2: Send mesh model data between provisioner and LPN: ok

Step 3: Power off and on again LPN

Result: LPN can not send data to provisioner again. It looks LPN can  not establish friendship again.

Question: 

1. How to make LPN establish friendship after reboot? Do we need to store CDB information in LPN ?

2. What should I do in source code of provisioner ? (provisioner may reboot and establish provisioned nodes )

Thank you!

Thanh