What are some factors or requirements when designing an Active Directory Infrastructure? How do you gather the

Question:

What are some factors or requirements when designing an Active Directory Infrastructure? How do you gather the requirements for the design? Explain in approximately in two paragraphs.
Active Directory Replication Topology How do you choose the appropriate Active Directory replication topology for your company? What kinds of problems do you want to prevent? Give examples. Explain in approximately in two paragraphs