Referring to 
Step 2a-1: the second node sends a third message (which may also be referred to as a first data configuration message or other name) to the first node, for example, which is a central unit of the donor node or a control plane part of a central unit of the donor node, or a user plane part of a central unit of the donor node, and the first node is a distributed unit of the donor node. This third message is used to configure the first node for transmission of a data packet that is sent to a relay node (or a distributed unit of the relay node), i.e., the destination address of the data packet is the address of the relay node, for example, which may be the migrated node or a child node of the migrated relay node (the child node may be directly connected to the migrated relay node or may be connected to the migrated relay node through one or more nodes), and this third message may include at least one of the following information: