Understanding Switch Fabrics Part 1

To begin understanding Ethernet Fabrics, as seen through the eyes of Brocade, it’s important to know about classic ethernet switches, how they have traditionally been provisioned, and why the way we have done this in the past isn’t really the best way to do things.Let’s begin with figure 1 where we see a classic ethernet switch topology. In the topology let’s assume that we have traffic flowing between Switch A and Switch E. What we know about traditional technology is that for Layer 2 we cannot have a loop. Why? Because loops are bad. Why are they bad? Because of the design of ethernet. So, with that in mind, we are to understand that we have a loop between B, C, and D, and that somehow that loop will be blocked. How so? Spanning Tree Protocol. It doesn’t matter if its Rapid, Multiple, or any other flavor. The fact is that it’s going to be running and it’s going to remove the loop.

Fabric1 1

So now looking at figure 2 we can see the topology now that spanning tree has done its thing.


What’s the big deal?

Why doesn’t this work well in todays networks? Because the convergence time when STP flips links takes too long and you wind up losing packets, and because you’re wasting a perfectly good link.

So how does an Ethernet Fabric work and why does it solve my problems?

At a high level, an Ethernet fabric provides a more flexible interconnected network between individual switches. We'll just call this a Fabric. Switches that form a fabric create something called a virtual cluster which consists of physical switches. This is seen in figure 3, where switches B, C, and D have formed a virtual cluster, or fabric. You can now see how the classic ethernet switches, A and E, are sitting outside of the virtual cluster, connected to the cluster by an edge port, and the Inter-Switch Links of the cluster are now called Fabric Ports.


Fabric Ports are transparent to the classic ethernet switches which are connected to edge ports. The fabric and fabric ports appear as just a single switch to external classic ethernet switches. This is reminisce of how MST domains are seen to CST if you're paying attention. A key difference however, is that we are no longer running STP in the Virtual Cluster.

So what about the classic ethernet switches that are still running STP? Now, BPDU's used by the classic ethernet switches, are transported transparently through the Fabric. Understanding these basic concepts bring us to the point where we need to begin looking at how a switch operates and how a cluster is formed. In the next post we’ll take a look at the Brocade VDX switch and it’s two operational modes, Classic Mode and VCS mode.