I was talking with one of my CCNA Students recently and he asked about the 3-Layer Heirarchical Model that we teach; Access, Distribution, Core. He asked if this is what everyone does. It’s a good question, because when a student is just starting out they generally only know one of two things:
- What they do at work.
- What the training manual tells them.
As we were talking I could tell that this student didn’t know what was happening at work, but knew that they were running something called SPB. SPB is shortest path bridging. SPB would be used to replace the spanning-tree protool in a data-center Leaf-Spine design. This design is an altertanative to the access/distribution/core design that CCNA students have been learning about for a long time. That’s not to say that the access/distribution/core is not a valid and relevant design. But it has some drawbacks.
So, here’s a little comparison between two different designs.
Access/Distribution/Core vs Leaf-Spine
Take first the image of the access/distribution/core design seen below. This design depicts the access layer at the bottom, the distribution or aggregation in the middle, and the core at the top. The core would take us to other places in the network, like the Internet-edge or WAN. If you are connected to the access-layer and traversing the uplinks, you have two paths to take. Spanning-Tree manages the loop between the access and the distribution layers. This is what a CCNA student learns.
Now note the Leaf-spine design seen below. In this design the access-layer is formed as a mesh, with one-hop between each access-layer device. In the above diagrame you are susceptible to bottlenecks where as the that becomes less likely in the leaf-spine design. From the bottom of the image you have the leaf, with the spine in the middle, and the core at the top.
There’s more to it of course. The fact that the leaf-spine can be Layer 2 or Layer 3 and so on, however for someone who is just learning what that means, this is probably good enough.
So, the short answer is no. The 3-Layer Hierarchical design is not what everyone uses, however CCNA candidate spend a bit of time examining some older networking concepts for a good reason. Learning concepts like the 3-Layer Hierarchical design helps you to understand why the new designs, like the leaf-spine, are implemented they way they are. Usually it’s to overcome limitations or drawbacks. Sometimes its to address an emerging need that was non-existant when the previous concepts were created. Whatever the case may be, its taught for a reason and you should learn it, even if you’re not using it at work.