Shortest Path Bridging (SPB), specified in the IEEE 802.1aq standard, is a computer networking technology intended to simplify the creation and configuration of networks, while enabling multipath routing.
It is the replacement for the older spanning tree protocols: IEEE 802.1D, IEEE 802.1w, IEEE 802.1s. These blocked any redundant paths that could result in a layer 2 loop, whereas SPB allows all paths to be active with multiple equal cost paths, provides much larger layer 2 topologies, supports faster convergence times, and improves the efficiency by allowing traffic to load share across all paths of a mesh network. It is designed to virtually eliminate human error during configuration and preserves the plug-and-play nature that established Ethernet as the de facto protocol at layer 2.
The technology provides logical Ethernet networks on native Ethernet infrastructures using a link state protocol to advertise both topology and logical network membership. Packets are encapsulated at the edge either in media access control-in-media access control (MAC-in-MAC) 802.1ah or tagged 802.1Q/802.1ad frames and transported only to other members of the logical network. Unicast, multicast, and broadcast are supported and all routing is on symmetric shortest paths.
The control plane is based on the Intermediate System to Intermediate System (IS-IS), leveraging a small number of extensions defined in RFC 6329.