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What is Spine-Leaf Architecture?

In data centers, networking is always pushing the limits of data rates and low latency. At the same time, redundancy is essential to prevent very costly downtime. How do you get the best of every world?

One viable answer is to utilize a spine-leaf architecture. It can simplify your networking design, reducing the total number of switches needed while solving the problems of latency and throughput.

The Basics of Spine-Leaf Architecture

Spine-leaf architecture is designed around the idea of rethinking data center architecture to provide better redundancy, less latency, and superior scalability. It aims to solve the problem by reducing the classic three-tier networking architecture to a two-tier system. The spine and leaf layers (more on each in a bit) form those two separate layers focusing more on east-west networking as opposed to north-south.

The leaf layer consists of aggregation switches while the spine layer is the core of the network. Every leaf switch is networked to every spine switch, but the spine switches are not interconnected directly to each other. The result is a semi-mesh that allows every endpoint device to reach every other endpoint device with the same number of midpoints.

Taking a closer look at the two layers, you can see how and why this architecture is appealing when speed and high-volume data are the most important concerns.

The Spine Layer

As the core layer, the spine layer handles routing for the network. Because each spine switch is connected to every leaf switch, you get a pseudo-mesh that allows the leaf switches to communicate with each other using minimal connection paths. At the same time, every switch in the spine serves as a complete redundancy for the whole network. This makes the network much more stable.

Despite that, the protocols that help route traffic through this network allow switches to connect via the shortest possible path. You don’t have to sacrifice data speeds for all of this redundancy.

The Leaf Layer

The leaf layer handles aggregating, and every endpoint device is connected to a leaf switch. The leaf switches exist in a mesh with each other, and that mesh is duplicated for every spine switch in the core of the network.

It is worth noting that the leaf layer is not as redundantly designed as the spine layer. Typically, endpoint devices will only connect to one leaf switch. When redundancy is important and viable, an endpoint device can connect to redundant switches as well.

Still, the design of the leaf layer is such that any leaf switch could go down, and only the endpoint devices dependent on that switch would be affected. The rest of the network would function just fine.

Spine-Leaf vs Traditional Cisco Network Architecture

Now that we’ve covered the spines and leafs, we can compare this idea to the traditional three-tier architecture. That will highlight some of the key differences and provide an option to look at pros and cons for both types of networking.

Three-Tier Architecture

A traditional three-tier network has a core layer, aggregation layer, and access layer. The spine-leaf design essentially removes the access layer from the equation.

With a three-tier design, the core layer handles routing, and core switches are usually networked with each other for the sake of redundancy (in a true mesh). The core layer then connects to the aggregation layer where each aggregation switch is typically connected to multiple core switches. This provides good redundancy, but in very large networks, it can be difficult to connect each aggregation switch to every core switch. This is the first major difference.

Then, there’s the access layer. Each endpoint device is connected to an access switch. Each access switch is connected to an aggregation switch along with one or two backups, depending on the network design.

The result of this is that an endpoint device might have to send data through five or more network nodes in a single data transaction. Those extra stops in the network add a lot of latency (when such a route is necessary) and make latency more unpredictable in general.

On top of that, three-tier networks typically use STP to send and receive data. This protocol has been around for a long time and is plenty powerful, but it struggles with optimizing data paths in highly redundant networks. Even though there are multiple viable paths for any data exchange, there are limited viable paths, and STP will often have to pause traffic to let information through congested network paths. Overall, in a crowded network, this slows down effective data rates.

Advantages of Spine-Leaf Designs

That brings us back to the spine-leaf design. It’s simpler, produces shorter networking paths, has greater overall redundancy, and scales better.

Much of this has been covered in previous sections, but let’s recap it all. First, spine-leaf can complete any data transaction with just three intermediary stops. In fact, it doesn’t make sense to ever add more stops than that in a network path, and so the protocols favored for spine-leaf architectures never do. This shorter path lowers latency and makes it more predictable. It also circumvents the STP issue of pausing traffic to let signals through a congested pathway. With spine-leaf configurations, there’s always an open pathway.

Scalability can be less obvious, but it’s easier to expand a spine-leaf network. Partly, this is because you have fewer layers, so you don’t have to add as many switches to increase the number of endpoint devices. It also ties back to the parallel networking design. You can simply add another aggregation switch, and plug it into each of your spines. That’s it. No complicated routing is necessary.

Overall, spine-leaf architectures do very well in data center environments. Enterprise networks that prioritize higher numbers of users over pure data rates and redundancy might still prefer three-tier architectures. In either case, understanding your options is essential for making informed decisions, and now you have a better idea of how spine-leaf networking functions.

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