The Deforestation of L2

A major staple of layer 2 has long been the combination of flood-and-learn Ethernet switches with some variant of the Spanning Tree Protocol. However, STP has significant shortcomings -- chiefly, that it throws away network capacity by removing links, and that it can be relatively slow to reconverge after topology changes. In recent years, attempts to rectify these shortcomings have been made by either making L2 look more like L3 (notably TRILL and SPB, which both incorporate L3-like routing) or by replacing L2 switches with "L3 switching" hardware and extending IP all the way to the host. In this paper, we examine an alternate point in the L2 design space, which is simple (in that it is a single data plane mechanism with no separate control plane), converges quickly, delivers packets during convergence, utilizes all available links, and can be extended to support both equal-cost multipath and efficient multicast.