Chapter 17. Multiple Spanning Tree Protocol

Spanning Tree Protocol (STP) is very important in Layer 2 (L2) networks, and its impact should be clearly understood when designing or even troubleshooting a network. If you’ve been around Cisco gear for the majority of your networking life, you’ve probably used Per-VLAN Spanning Tree (PVST) or Rapid-PVST (RPVST). In this chapter, I cover a form of STP that is becoming more common in large data centers: Multiple Spanning Tree (MST).

Data center networks have very different requirements than those of enterprise networks. I worked for a client that had Cisco 3750s in the core of a small data center. Things seemed to work great until the client added the 257th VLAN, and that’s when they learned that Cisco 3750s support STP only up to 256 VLANs. Bummer. I should note that this is not a knock on the 3750, but rather the idea that someone decided to use a device designed to be an office switch as a data center core.

I was brought in to help solve the problem, and after my recommendation of, “Buy data center-class chassis switches” was ignored, I looked for other options. That’s when I learned about MST.

Note

You might see both MST and MSTP (Multiple Spanning Tree Protocol) referenced. I tend to prefer MST because it’s easier to say, but EOS uses MSTP likely because they’re commands that affect how the protocol is used.

Arista switches can run a variety of STP types, but they default to MST. You can change the type by using the spanning-tree ...

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