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What is the point for Cisco to give its three-layer hierarchical model?
CCNA & CCENT
To be honest, this model looks pretty award to me. What is the advantage of it? Is lay2/layer3 model not enough to describe Cisco's devices' function?
It's necessary for planning data flow. The different layers on the model are responsible for different things, and you really need to plan it out if you want optimal traffic patterns. What roles the devices perform is just as important as what OSI layer they function on. A core router does something entirely different than a border router, and a distribution switch has an entirely different set of needs from that of an access switch.
It allows you to break down the network into different sections, making it easier to troubleshoot. As each layer does separate things, you can assign access to those layers based on roles.
If you're familiar with programming, they have the same concept. When you do programming, you don't code the entire thing in one go. You break it down into different layers - the presentation, business logic, and backend layer. This allows for collaboration, and make it easier to troubleshot/debug because you can pinpoint in which layer the problem exist and focus on that, instead of digging around the entire application.
good thread - great responses!
i would just add that in most smaller network environments often times the layers of the design model are collapsed for various reasons, making it hard to see a real-world instance of the theory. the exact opposite is true in large enterprise or higher ed networks - there you'll see a highly organized and structured design that fits very well with the theory. so, if you're interested in seeing or learning about design try to get some exposure to the latter type of environments or create one in your favorite simulator. when in doubt, just trust the many experts and years of experience - there IS a good reason for that design model - engineering just doesn't happen by chance
Many networks today still use the 3 tier architectural model for large campuses. For example;
A campus has several building, each building has multiple floors. Each floor has 4 IDF rooms with 3 Cisco switches. That makes 12 access switches per floor. Each floor has two distribution switches that each access switch connects to on that floor.
Each distribution switch connects to the core. Those cores connect to other core switches throughout the campus. An example i'll give you from a company I've worked with is;
Cisco 3750X-48 port PoE (stacked) at the access layer (layer 2 only)
Cisco 4507R's for the Distribution (which handles all vlan interfaces, routing, security and policies)
Cisco 6509E's for Core switches, 10gig to each distribution.
This is just an example and its getting quite common to not see this style of example as stacked switching and VSS has been replacing the traditional 3 tier model however many ISP's still use the 3 tier model. Commony you'll see WAN Edge, Regional distribution facilities and core facilities which commonly include ASR1000 Series at the wan edge, ASR9000 at the regional facilities and larger edge deployments and CRS-3's in the core with 100gig ethernet pipes.
There is no place like 127.0.0.1