LAN Topology

3-Tier network
Core layer
1. Connect routers to Distribution layer switches
2. Typically deployed in redundant pairs with downstream Distribution Layer switches connected to both
3. Because software policies slow switches down, they should be delegated to Distribution Layer switches
Distribution layer
1. Aggregation point between Access layer and Core layer
2. Often deployed in redundant pairs
3. Most software policies, such as QoS, are enabled at this layer
Access Layer
1. Bottom layer, connected to end hosts
2. Interfaces configured with low-trust, high-flexibility
3. Often no redundancy between access switches

Two-tier network
Used in smaller campuses that do not need the scalability of three separate layers
As the name suggests, the Core and Distribution layers are collapsed into a single layer that performs the functions of both

Two-tiered network
1. Sometimes called a Clos Network (for Charles Clos)
2. Often used with Cisco ACI
Designed for East-West scalability
1. All spine switches are connected to all leaf switches, BUT
2. Spine switches don't connect to spine switches
3. Leaf switches don't connect to leaf switches
Leaf Switches connect to EPGs (server groups) and Cisco APICs
1. APICs are typically deployed in clusters of three controllers
  1. Since they are not directly involved in forwarding traffic, they do not have to connect to every spine or leaf
  2. APICs connect to leaf nodes to reduce strain on spine nodes and free up ports on spine nodes
Ensures 2-hop maximum for any server to talk to another one
1. This means there is consistent latency and reliable communication
  Cisco ACI is basically just Spine-Leaf Architecture

Typically provided by a single device, often called a "home router" or "wireless router"
This one device can provide:
1. Router
2. Switch
3. Firewall
4. Wireless AP

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