Hi, What is the impact if there is no site boundary defined in SCCM? boteasy.

No auto site assignment and all DPs are treated as remote/slow (IOW you cannot control where the content is coming from).

For site assigment, we will install the sccm clinet by setting the site code manually. For the DPs, we have only 1 DP in the SCCM site and thus the client should download the content from that DPs only. Is it correct?

Yes it is, but why don't you use boundaries?