A three-tier campus has an access layer where devices attach, a distribution layer that aggregates the access switches, and a core that joins the distribution blocks. In a single building the core can be collapsed into the two distribution switches. A spine-and-leaf data center connects every leaf switch to every spine switch, spreads server-to-server traffic over all paths, and grows by adding more switches of the same type.
The three tiers
Cisco's campus design overview describes the access layer as the first tier, or edge, of the campus, where end devices attach.
The distribution layer is an aggregation point for all the access switches.
The core provides a very limited set of services and is designed to be highly available. In the three-tier model it joins the distribution blocks.
Collapsed core
Where the campus is contained within a single building, or adjacent buildings with enough fiber, Cisco says it is possible to collapse the core into the two distribution switches. The result is a two-tier design called a collapsed core.
A separate physical core becomes worth adding as the network grows and changes begin to affect the core devices.
Spine and leaf in the data center
RFC 7938 explains the shift. Traditional tree topologies suited north-south traffic entering and leaving the data center. When more bandwidth was needed, it was added by scaling up: bigger line cards or switches with more ports. Modern data centers carry heavy east-west traffic between servers, and scaling a tree for that becomes too expensive or impossible.
Its first requirement is a topology that scales horizontally, by adding more links and devices of the same type without upgrading the existing ones. A folded Clos topology meets it. RFC 7938 notes this is often called a leaf and spine network: the spine is the middle stage and the leaf is the input/output stage that servers connect to.
Each leaf connects to every spine. Traffic from server to server is load balanced over all available paths using ECMP.
Where the exam catches people
Believing a spine-and-leaf fabric grows by buying bigger spine switches: RFC 7938 says it scales horizontally by adding more devices of the same type.
Believing the core layer is where user devices and heavy policy belong: Cisco says the core offers a very limited set of services and is built to be highly available; devices attach at the access layer.
Believing a collapsed core removes the access layer: it merges core and distribution into the two distribution switches; access stays.
Believing a tree topology spreads server-to-server traffic over every path: that is a property of the Clos (spine and leaf) design using ECMP.