What are two key design principles when using a hierarchical core-distribution-access network model?

The hierarchical model provides:

A: Fault isolation by limiting failures to specific layers.

D: Modularity and flexibility, making it easier to implement changes without affecting the entire network.

Why other options are incorrect:

B: Design typically starts at the access layer upwards.

C: Server access is handled at the distribution or access layer, not the core.

E: Security is handled at access/distribution layers, not primarily at core layer.