This is a general question which the interviewer may ask early in the interview to begin the conversation, learn more about your background, and collect some information they can use for subsequent questions. When discussing challenges, your answer should align with the challenges the organization with which you are interviewing may be facing. You can learn about these by researching the organization, its network infrastructure, and any challenges it talks about. Resources for the research include its website, public financial documents, or discussions in user forums. Once you discover these, you can answer this question by describing a similar challenge and the ways resolved them.
"One of the biggest challenges I have encountered in my career as a network administrator is overcoming vendor lock-in. Many organizations similar to yours will elect to implement technology from a single vendor whose products and solutions are leading edge when they choose to work with them. However, this industry evolves quickly, and other solutions come to the market regularly. It is often difficult to transition between the older solutions, which are vendor proprietary and newer ones, which have better features. I've overcome this by choosing to work with open source systems and ensuring that the vendors are compatible with other products before selecting them."