Request: What questions do you ask candidates to try to understand their grasp of network design?

I'm sure more than a few of you saw my post this week about how to handle interviews, but in the process of giving a few this week, I realized I'd like to refine the scenario I present when interviewing about design.

To clarify, I'm trying to determine the candidate's ability to approach a given scenario and talk about one or more of the following:

• How would you organize addressing?
• How would you provide reachability in the provided topology? What routing protocols would you use? How would you handle internal and external connectivity?
• As the network grows, what are some potential issues with the responses you've given? How might you change your answer if scalability and/or reliability are paramount?
• What are the caveats to your proposals? Are there any tradeoffs I should be aware of?

Unfortunately I only have about 45 minutes to get insight into how candidates approach design. That means I have to try to strike a balance of depths vs. time. I feel like I've been able to do that well in the other areas where I normally conduct technical interviews, but so far I've been a little frustrated at how much time gets eaten up clarifying things when talking about design. I want to make sure a given candidate can talk about why they're proposing a specific solution, because there's almost never just one "right" way to do things, but I only have a limited amount of time to spend working with them on a given scenario.

I have a feeling my current scenario is too open ended. I'm providing candidates with a couple of small "data centers" (where services are hosted) that each have their own internet connection. I provide a L2 link between sites (to try to keep things simple) and have asked candidates how they would go about designing such a network in terms of the points mentioned above. I feel like I need to put more "rails" on the question, either by providing more initial assumptions/constraints or by making the initial scale of the question smaller.

Have any of y'all asked questions like this in interviews? I'd love to hear about approaches/typologies, and how they worked out (successful or not).