Enterprise Architectures and Traffic Flow
Unit 1: Networking Foundations. Lesson 3 of 49 in NetPlus Academy.
Large networks are organized in layers so that each layer has one job.
The three-tier hierarchy
- Access layer: where end devices plug in. Access switches provide ports, PoE, and VLAN assignment.
- Distribution layer: aggregates access switches, applies policy such as ACLs and QoS, and routes between VLANs.
- Core layer: a fast, simple backbone that moves traffic between distribution blocks. No policy, just speed.
Collapsed core
Small and medium networks merge the core and distribution layers into one pair of devices. Cheaper and simpler, at the cost of scalability.
Spine and leaf
Data centers use a two-tier fabric. Every leaf switch (where servers connect) links to every spine switch, and leaves never connect to leaves. Any server can reach any other in exactly two hops, so bandwidth is predictable and the fabric grows by adding spines or leaves. It is built for traffic that moves sideways between servers.
Traffic flows
- North-south: traffic entering or leaving the data center, for example a user reaching a web server. It crosses the perimeter and the firewall.
- East-west: traffic between servers inside the data center, for example a web server querying a database. In modern applications most traffic is east-west, which is why spine and leaf exists.
Enterprise Architectures and Traffic Flow, explained slowly
Why layers of switches?
A network with 40 people can be one switch. A network with 4,000 people cannot, so designers split it into tiers where each tier has one job. Think of roads: neighborhood streets, main roads, and the highway.
The three-tier design
[ Core ] highway: fast, no stoplights
/ \
[ Distribution ] [ Distribution ] main roads: rules, routing between VLANs
/ | \ / | \
[Acc] [Acc] [Acc] [Acc] [Acc] [Acc] neighborhood streets: where devices plug in- Access layer: the switches your PC, phone, and access point plug into. They give you a port, power over Ethernet, and a VLAN assignment.
- Distribution layer: gathers up the access switches and applies policy. Access control lists, quality of service, and routing between VLANs live here.
- Core layer: moves traffic between distribution blocks as fast as possible. No policy, no filtering, just speed. Anything that slows the core slows everyone.
Collapsed core
A medium-sized company does not need three tiers. It merges the core and distribution into one pair of big switches. Cheaper and simpler. The trade-off is that it will not grow as far.
Spine and leaf
Data centers have a different problem: servers talk to other servers constantly. The three-tier design sends that traffic up and down, which adds hops and unpredictability. Spine and leaf fixes it with a rule: every leaf connects to every spine, and leaves never connect to leaves.
[Spine 1] [Spine 2]
/ | \ / | \
/ | \ / | \
[Leaf A][Leaf B][Leaf C] (each leaf has a link to EVERY spine)
| | |
servers servers serversAny server to any other server is exactly two hops: up to a spine, down to a leaf. Need more capacity? Add a spine. Need more ports? Add a leaf. Bandwidth stays predictable because every path is the same length.
North-south and east-west
Picture the data center as a box.
- North-south traffic enters or leaves the box: a user on the internet loading a web page. It crosses the perimeter firewall.
- East-west traffic moves sideways inside the box: the web server asking the database server for data. In modern applications most traffic is east-west, which is exactly why spine and leaf exists.
How the exam asks it
- "Where do ACLs and inter-VLAN routing belong" means distribution.
- "Every access switch connects to every upper switch, and switches in the same tier never connect": spine and leaf.
- "Small business wants fewer devices": collapsed core.
- "Traffic between servers in the same data center": east-west.
What to memorize
- Access plugs in, distribution decides, core moves.
- Spine and leaf: two hops, every leaf to every spine, built for east-west.