Abstract-Enterprise networks are large and complex, and their designs must be frequently altered to adapt to changing organizational needs. The process of redesigning and reconfiguring enterprise networks is ad-hoc and error-prone, and configuration errors could cause serious issues such as network outages. In this paper, we take a step towards systematic evolution of network designs in the context of virtual local area networks (VLANs). We focus on VLANs given their importance and prevalence, the frequent need to change VLAN designs, and the time-consuming and error-prone process of making changes. We present algorithms for common design tasks encountered in evolving VLANs such as deciding which VLAN a new host must be assigned to. Our algorithms trade off multiple criteria such as broadcast traffic costs, and costs associated with maintaining spanning trees for each VLAN in the network, while honoring correctness and feasibility constraints on the design. Our algorithms also enable automatic detection of network-wide dependencies which must be factored when reconfiguring VLANs. We evaluate our algorithms on longitudinal snapshots of configuration files of a large-scale operational campus network obtained over a two year period. Our results show that our algorithms can produce significantly better designs than current practice, while avoiding errors and minimizing human work. Our unique data-sets also enable us to characterize VLAN related change activities in real networks, an important contribution in its own right.

Virtual Local Area Networks (VLANs) are extensively used in enterprise networks. However, their configuration remains an ad-hoc, complex and error-prone process today. We believe that to eliminate these difficulties, there is need for automation tools, and also need for visualization tools. In this paper, we report on our experience building a VLAN management toolkit, which automates and visualizes common VLAN configuration tasks. We begin by describing common misconfigurations, and their impact on network performance and security. We next present a set of algorithms that automate the VLAN configuration tasks. These algorithms form the back end of the toolkit. The front end of the toolkit consists of an interactive graphical user interface which provides visualization of VLAN operations at multiple granularities, and can be accessed remotely from a web browser. We are in the process of deploying the toolkit at a large campus network which has thousands of switches, and around 800 VLANs. Our initial operational experience shows that the toolkit is effective in both automating configuration tasks, and identifying common misconfigurations. In particular, we have found that (i) more than 40% of the VLANs in the network have redundant links that may lead to security and performance issues. (ii) more than 30% of the VLANs in the network have missing links which may result in connectivity issues and (iii) the root-bridge placements of more than 30% of the VLANs are not optimum, which again may result in performance issues. We believe these insights highlight the benefit and importance of such a toolkit.