802.1Aq

CARRIER SCALE ETHERNET

Shortest Path Bridging: Efficient Control of Larger Ethernet Networks
David Allan, Ericsson Peter Ashwood-Smith, Huawei Nigel Bragg, Ciena János Farkas, Ericsson Don Fedyk, Alcatel-Lucent Michel Ouellete, Huawei Mick Seaman, Consultant Paul Unbehagen, Alcatel-Lucent

ABSTRACT
This article provides an overview of IEEE 802.1aq shortest path bridging and outlines someapplication scenarios that will benefit from the new capabilities SPB offers. SPB is built on the IEEE 802.1 standards, and inherits unaltered the existing OAM and data plane scalability enhancements, such as the MAC-in-MAC forwarding paradigm. SPB introduces link state control for bridge networks, thus improving control plane scalability, network bandwidth utilization, and control of theforwarding paths. Furthermore, SPB minimizes latency by forwarding frames on the shortest path. Network-wide load balancing is also supported by spreading the traffic on multiple equal cost paths in a user controllable manner. Thus, SPB provides enhanced control for Ethernet networks in metro, RAN backhaul, or data center environments.

INTRODUCTION
The traditional Ethernet bridging and data link layerhas served well for several decades now and forms a nearly universal layer 2 infrastructure for many protocols including IP/multiprotocol label switching (MPLS). Furthermore, Ethernet bridging used as a service layer provides end-to-end Ethernet connectivity, and is complemented by robust and fully featured operations, administration, and maintenance (OAM) capabilities. Much of the success ofEthernet is due to its plug-and-play nature, which is highly valued by consumers and operators alike. This desire to preserve simplicity also led to the adoption of relatively simple control planes in the form of the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP). These proto-

cols automatically maintain the active topology required for learning bridges, while requiringminimal configuration and storage. Multiple Spanning Tree Protocol (MSTP) was added to allow multiple spanning trees to be constructed over small meshes of Ethernet switches. The primary shortcoming of spanning trees is that they do not utilize all links in certain topologies. The resulting connectivity is inefficient for traffic that does not terminate at the root of the spanning tree. Furthermore,spanning tree protocols implement the transactional distance-vector class of algorithm instead of a proper topology database, which adversely impacts the convergence time of an Ethernet network after a topology change. To date, this has constrained the scale and utility of pure Ethernet networks. The addition of 802.1ah provider backbone bridging (PBB) [1] to Ethernet highlighted the need for a newcontrol protocol to address these issues. Any such control protocol would be required to maintain all of the key architectural properties of Ethernet specified by IEEE 802.1 and make minimal changes of detail only where essential so that the huge body of existing work and implementations could be leveraged. A further key requirement was to use minimum cost paths in an arbitrary mesh and in ways notlimited to a single shortest path between any two points. Shortest path trees (SPTs) can be built to both support Ethernet 802.1Q and leverage the new medium access control (MAC)-in-MAC encapsulation (first specified for PBB) for larger networks. MAC-in-MAC is ideally suited for isolation of layer 2 virtual private networks (VPNs) and, when combined with the 802.1aq shortest path bridging (SPB)[2, 3] control plane, truly extends the scalability of Ethernet by several orders of magnitude. It was also a requirement to support the VLAN and provider bridge (PB)

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0163-6804/10/$25.00 © 2010 IEEE

IEEE Communications Magazine • October 2010

data path, albeit at smaller scales, so that existing inexpensive packet forwarding applicationspecific integrated circuits (ASICs) could...