Collapsed core architectures differ from 3 tier design by combining the function of the Core and Distribution layers. The wording on this subject is very precise and I failed several times to put it as well as it is described below, so I have just copy and pasted it from Cisco’s website.
From Cisco’s website:
A typical hierarchical enterprise campus network design includes the following three layers:
- Core layer: Provides optimal transport between sites and high-performance routing. Due the criticality of the core layer, the design principles of the core should provide an appropriate level of resilience that offers the ability to recover quickly and smoothly after any network failure event with the core block.
- Distribution layer: Provides policy-based connectivity and boundary control between the access and core layers.
- Access layer: Provides workgroup/user access to the network.
The two primary and common hierarchical design architectures of enterprise campus networks are the three-tier and two-tier layers models.
This design model, illustrated in Figure 3-1, is typically used in large enterprise campus networks, which are constructed of multiple functional distribution layer blocks.
This design model, illustrated in Figure 3-2, is more suitable for small to medium-size campus networks (ideally not more than three functional disruption blocks to be interconnected), where the core and distribution functions can be combined into one layer, also known as collapsed core-distribution architecture.