3-Tier, 2-Tier and Spine-Leaf

The three-tier design is where enterprise hierarchy begins, and you already know it from your CCNA.
This review covers it first, then the two-tier and the spine-and-leaf, the three designs ENCOR expects you to recognize on sight.

Three-tier means each layer has one clear job.

The Three Layers

The hierarchy splits the network into three layers:

• Access is where users and devices plug in, mainly Layer 2.

• Distribution aggregates the access switches and applies policy.

• Core is the high-speed backbone between distribution blocks.

Figure 1 – The three-tier model: Access, Distribution, and Core

One access switch with its distribution forms a repeatable unit, called a switch block.
You scale the network by adding blocks, not by redesigning it.

Access Layer

The Access Layer is the edge where users connect.
Laptops, IP phones, printers, and access points all plug in here.

Figure 2 – The Access Layer connects end-user devices

It runs mostly at Layer 2, forwarding by MAC address.
It also delivers edge services and security: VLANs, STP, and Port Security.

Distribution Layer

The Distribution Layer aggregates every access switch in a block.
It works at Layer 3, routing between VLANs and toward the core.

Figure 3 – The Distribution Layer aggregates and routes

This is where the Layer 2 to Layer 3 boundary sits.
Policy lives here too: ACLs, QoS, and route summarization toward the core.

Core Layer

The Core Layer is the backbone.
Its only job is to move traffic between distribution blocks as fast as possible.

Figure 4 – The Core Layer is the high-speed backbone

Keep the core simple.
You avoid ACLs, NAT, and filtering here, because every extra operation adds latency to traffic passing through.

In a multi-building campus, each building keeps its own access and distribution.

Figure 5 – Multiple buildings interconnected through a centralized core

The core sits in the data center and links every block through redundant high-speed paths, usually fiber with EtherChannel.

Here are the three layers side by side:

Table 1 – Three-tier roles and technologies

Now shrink the three-tier.
In smaller sites a dedicated core is wasted hardware, so you merge two layers into one.