IT networks, which started as simple setups with a few switches and PCs, have evolved significantly. Today, they stretch vast distances, linking hundreds of thousands of computers and devices worldwide. The Internet, a prime example of this evolution, is a network that we all use and comprehend, underscoring the magnitude of this transformation. 

The swift progression of technology has brought forth a diverse array of technologies, protocols, and mechanisms to meet various needs and overcome numerous challenges. However, many of these solutions are complex and interdependent, resulting in a layered and intricate network architecture. This complexity makes traditional data networks increasingly challenging to construct, operate, and troubleshoot, often leading to difficulties in identifying and resolving issues.

What is Shortest Path Bridging?

Shortest Path Bridging (SPB) is a network technology that optimizes traffic delivery by ensuring it takes the shortest available path. This feature makes it an excellent solution for virtualization in carrier-grade networks and data centers, enhancing their efficiency and performance.

Shortest Path Bridging (SPB) is a transformative computer networking technology initially specified by the IEEE 802.1aq standard in 2012 and later incorporated into the IEEE 802.1Q standards document. It was designed to address the inherent challenges of the spanning-tree protocol, streamline the creation and configuration of Ethernet networks, enable multipath traffic routing, and support large-scale virtualization.

SPB revolutionized networking by replacing the myriad of protocols in the traditional network protocol stack with a single protocol: IS-IS (Intermediate System to Intermediate System). This protocol, which serves as the control plane or the “brain” of SPB, is a robust, extensible, and well-established routing protocol similar to OSPF (Open Shortest Path First), although IS-IS predates OSPF.

IS-IS, being the brain of SPB, performs two crucial tasks:

  1. Building the Network Topology and Calculating Shortest Paths:
    • Nodes or switches exchange hello messages to form adjacencies and share information about the network topology.
    • Each switch uses this information to determine the shortest path to every other switch in the network by employing the well-known Dijkstra’s Shortest Path First (SPF) algorithm.
  2. Operational Announcements and Tasks:
    • IS-IS disseminates information necessary for various operational tasks, such as building new virtual networks or extending existing ones to new switches.
    • It also announces the availability of multicast streams, notifies the multicast source node when a receiver requests a copy of the stream, and performs many other tasks essential for network operation.

By consolidating the control of network operations under IS-IS, SPB simplifies network management, enhances scalability, and improves the efficiency and reliability of Ethernet networks. This innovation makes SPB invaluable for modern, large-scale, and virtualized network environments.

Shortest Path Bridging (SPB) also features a data plane for transporting traffic across the network. While fundamentally Ethernet, SPB incorporates several key modifications:

  1. MAC-in-MAC Encapsulation: Traffic forwarded over an SPB network is encapsulated in a new Ethernet frame, MAC-in-MAC encapsulation, defined by the IEEE 802.1ah standard. This encapsulation provides hierarchical Ethernet addressing, among other benefits.
  2. No Flooding-and-Learning: Unlike traditional Ethernet, SPB does not rely on flooding and learning mechanisms (details on how this is achieved will be discussed later).
  3. Multicast Instead of Broadcast: In SPB networks, one-to-many traffic is handled through multicast rather than broadcast, eliminating unnecessary broadcast traffic.
  4. Loop-Free Traffic: A critical feature of the SPB fabric network is that traffic never loops, allowing for almost any physical topology without concern for loops. This is achieved through two mechanisms:
    • Control Plane: Utilizes shortest path forwarding.
    • Data Plane: Employs a reverse path forwarding check, where a switch will only forward a frame if it arrives on a port that is on the shortest path back to the source. If not, the frame is dropped.

Aside from these changes, all the foundational principles of Ethernet still apply.