STP - Shortest Path Bridging (SPB)

Shortest Path Bridging (SPB), specified in the IEEE 802.1aq standard, is a computer networking technology designed to simplify the creation and configuration of networks, while enabling efficient multipath routing. It serves as a replacement for the older Spanning Tree Protocols like STP (IEEE 802.1D), Rapid STP (IEEE 802.1w), and Multiple STP (IEEE 802.1s), which blocked redundant paths that could result in a Layer 2 loop. In contrast, SPB allows for all paths to be active and supports larger Layer 2 topologies with faster convergence times.

Note that SPB is always deployed in conjunction with the IS-IS routing protocol. The underlying principles of shortest path bridging could theoretically be implemented with other control plane routingprotocols. However, the standardization and widespread adoption of SPB are specifically linked with IS-IS, making it the de facto control plane protocol for SPB as per the IEEE 802.1aq standard. While SPB is not exclusively tied to IS-IS in terms of theoretical possibilities, its practical implementation, standardization, and current deployments are closely integrated with the IS-IS protocol.

Key aspects of SPB include:

  1. Multipath Routing: SPB enables the use of all available physical connections in the network, allowing traffic to load-share across all paths of a mesh network. This results in better utilization of network resources and improved efficiency.

  2. Simplified Network Configuration: SPB maintains the plug-and-play nature of Ethernet, virtually eliminating human errors during network configuration. It simplifies the process of setting up networks, making them more accessible and manageable.

  3. Control Plane: SPB uses the IS-IS protocol as its control plane, with a few extensions defined in RFC 6329 (linked below). This control plane has a global view of network topology, which facilitates faster restoration of connectivity after failures and ensures that only directly affected traffic is impacted during restoration.

  4. Virtualization Support: SPB is particularly suitable for applications that involve virtualization in both Carrier and Enterprise spaces. It offers a perfect emulation of a transparent Ethernet LAN segment, which is essential for server virtualization and connectivity virtualization.

  5. Ethernet VPN Services: In the Carrier space, SPB is used to deliver Ethernet VPN services to Enterprises over common Carrier infrastructure. It ensures complete transparency for customer Ethernet services and isolation between different customers' traffic.

  6. Operational Advantages: SPB supports rapid restoration of broadcast and multicast connectivity. It floods all required information in the SPB extensions to IS-IS, allowing unicast and multicast connectivity to be installed in parallel.

  7. Multicast Traffic Management: SPB offers efficient multicast replication models, where packets are replicated only at fork points in the shortest path tree, and also supports head-end replication models for different traffic and service requirements.

  8. Ethernet Operations, Administration, and Management (OA&M): SPB builds on existing Ethernet OA&M, ensuring that its unicast and multicast packets for a given VLAN follow the same forward and reverse path and use standard Ethernet encapsulations.

Shortest Path Bridging significantly enhances the capabilities of Ethernet networks, providing a more robust, efficient, and scalable solution than traditional spanning tree protocols. It's particularly effective in large-scale, complex network environments where efficient use of all available paths, rapid convergence, and ease of management are critical.


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