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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 a 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

n 4 March 2006 the working group posted 802.1aq draft 0.1.

In December 2011 Shortest path bridging (SPB) was evaluated by the JITC and approved for deployment within the US Department of Defense (DoD) because of the ease in integrated OA&M and interoperability with current protocols. On March 2012 the IEEE approved the 802.1aq standard.

In May 2013 the first public multi-vendor interoperability was demonstrated as SPB served as the backbone for Interop 2013 in Las Vegas.

The 2014 Winter Olympics was the first "fabric-enabled" Games using Shortest Path Bridging (SPB) "IEEE 802.1aq" technology. During the games this fabric network was capable of handling up to 54,000 Gbit/s (54 Tbit/s) of traffic. In 2013 and 2014 SPB was used to build the InteropNet backbone with only 1/10 the resources of prior years. During Interop 2014 SPB was used as the backbone protocol which can enable Software-defined networking (SDN) functionalities


Both SPBV and SPBM inherit key benefits of link state routing:

  • the ability to use all available physical connectivity, because loop avoidance uses a Control Plane with a global view of network topology
  • fast restoration of connectivity after failure, again because of Link State routing's global view of network topology
  • under failure, the property that only directly affected traffic is impacted during restoration; all unaffected traffic just continues
  • rapid restoration of broadcast and multicast connectivity, because IS-IS floods all of the required information in the SPB extensions to IS-IS, thereby allowing unicast and multicast connectivity to be installed in parallel, with no need for a second phase signaling process to run over the converged unicast topology to compute and install multicast trees

Virtualisation is becoming an increasingly important aspect of a number of key applications, in both Carrier and Enterprise space, and SPBM, with its MAC-in-MAC datapath providing complete separation between Client and Server layers, is uniquely suitable for these.

"Data Centre virtualisation" articulates the desire to flexibly and efficiently harness available compute resources in a way that may rapidly be modified to respond to varying application demands, without the need to dedicate physical resources to a specific application. One aspect of this is server virtualisation. The other is connectivity virtualisation, because a physically distributed set of server resources must be attached to a single IP subnet, and modifiable in an operationally simple and robust way. SPBM delivers this; because of its client-server model, it offers a perfect emulation of a transparent Ethernet LAN segment, which is the IP subnet seen at Layer 3. A key component of how it does this is implementing VLANs with scoped multicast trees, which means no egress discard of broadcast/unknown traffic, a feature common to approaches that use a small number of shared trees, hence the network does not simply degrade with size as the percentage of frames discarded goes up. It also supports "single touch" provisioning, so that configuration is simple and robust; the port of a virtual server must simply be bound locally to the SPBM I-SID identifying the LAN segment, after which IS-IS for SPB floods this binding, and all nodes that need to install forwarding state to implement the LAN segment do so automatically.

The Carrier-space equivalent of this application is the delivery of Ethernet VPN services to Enterprises over common Carrier infrastructure. The required attributes are fundamentally the same; complete transparency for customer Ethernet services (both point-to-point and LAN), and complete isolation between one customer's traffic and that of all other customers. The multiple virtual LAN segment model provides this, and the single-touch provisioning model eases carrier operations. Furthermore, the MAC-in-MAC datapath allows the carrier to deploy the "best in class" Ethernet OAM suit (IEEE 802.1ag, etc.), entirely transparently and independently from any OAM which a customer may choose to run.

A further consequence of SPBM's transparency in both dataplane and control plane is that it provides a perfect, "no compromise" delivery of the complete MEF 6.1 service set. This includes not only E-LINE and E-LAN constructs, by also E-TREE (hub-and-spoke) connectivity. This latter is clearly very relevant to Enterprises customers of Carrier VPN services which have this network structure internally. It also provides the carrier with the toolkit to support geo-redundant broadband backhaul; in this applications, many DSLAMs or other access equipments must be backhauled to multiple BNG sites, with application-determined binding of sessions to a BNG. However, DLSAMs must not be allowed to communicate with each other, because carriers then lose the ability to control peer-to-peer connectivity MEF E-TREE does just this, and further provides an efficient multicast fabric for the distribution of IP-TV.

SPBM offers both the ideal multicast replication model, where packets are replicated only at fork points in the shortest path tree that connects members, and also the less state intensive head end replication model where in essence serial unicast packets are sent to all other members along the same shortest path first tree. These two models are selected by specifying properties of the service at the edge which affect the transit node decisions on multicast state installation. This allows for a trade-off to be made between optimum transit replication points (with their larger state costs) v.s. reduced core state (but much more traffic) of the head end replication model. These selections can be different for different members of the same Individual Service ID (I-SID) allowing different trade-offs to be made for different members.

Figure 5 below is a quick way to understand what SPBM is doing on the scale of the entire network. Figure 5 shows how a 7-member E-LAN is created from the edge membership information and the deterministic distributed calculation of per source, per service trees with transit replication. Head end replication is not shown as it is trivial and simply uses the existing unicast FIBs to forward copies serially to the known other receivers.

Operations and management

802.1aq builds on all existing Ethernet Operations, administration and management (OA&M). Since 802.1aq ensures that its unicast and multicast packets for a given virtual lan (VLAN) follow the same forward and reverse path and use completely standard 802 encapsulations, all of the methods of 802.1ag and Y.1731 operate unchanged on an 802.1aq network.

See IEEE 802.1ag and ITU-recommendation Y.1731 (external link below).

Source: Wikipedia, Google