Shortest Path Bridging is the most recent of this series of evolutionary steps, and is arguably one of the 3 or 4 most significant enhancements in Ethernet's history. Until SPB, Ethernet had retained its original control mechanisms, and these are now distinctly behind the state of the art in their properties. SPB refreshes this component of Ethernet, by taking the existing data path technology practically unaltered, and marrying it to a significant extension of the state of the art in distributed control planes, link state routing. The book both explains both the "what" and the "why" of the standard. The intent is to provide a sense of the relative simplicity of 802.1aq, in terms of the small number of moving parts required to achieve what it does, and why those choices were made. It goes into what were elective decisions and what decisions were dictated by the design goals. It does this by using a multipart approach to the book. The first is a "what it is" description, intended to provide an overview of SPB. The second is separated out, and uses a narrative form to describe the design process and decisions that led to SPB, in order to provide further context in understanding the first part. The book is rounded out with applications and potential futures for the technology to suggest where it could go.