Shortest Path Bridging (SPB)

If you want to know about Shortest Path Bridging (SPB), you should understand Bridging, Provider Bridging and Provider Backbone Bridging (PBB) first. Bridging is the beginning to understand tagging, Provider Bridging is the same than Bridging but with two tags to solve the limitation of 4096 VLANs, and Provider Backbone Bridging is an encapsulation method to hide customer MAC addresses to the backbone to save money in expensive core switches with large TCAM memories. However, we still have issues with regard to loop avoidance, convergence delays, North-South traffic, performance, etc in this kind of Layer 2 networks. As a result to solve these issues, SPB is here.

Shortest Path Bridging or SPB is the standard 802.1aq by IEEE and the main different with regard to Provider Backbone Bridging is the use of the dynamic routing protocol IS-IS in the control plane instead of the Spanning Tree Protocol (STP). This is a good news because we'll have many advantages with SPB like reliable layer 2 topologies, real layer 2 multipath forwarding, high scalability topologies with more than 1000 nodes, mesh topologies, faster convergence times, multiple Equal Cost Paths, hiding customer frames to the backbone, VLAN extension, no loops, high availability and better performance.

STP is a protocol for loop avoidance with North-South traffic where we'll have a root switch in the core with multiple links to others switches for high availability. However, all links are not used at the same time because some of them will be in the blocking state. Maybe, you are thinking about PVST+ or Rapid PVST+, which can block links by VLANs, and this is a way to use all links at the same time. Nevertheless, Spanning Tree design, implementation and troubleshooting is more hard and difficult than SPB, while Shortest Path Bridging is easy to manage, all links are active at the same time with microseconds (by hardware) or miliseconds (by software) of convergence time and also real layer 2 multipathing for East-West traffic.

SPB encapsulation is like PBB encapsulation where customer frames, even the Customer VLAN (C-VID), are encapsulated inside Backbone frames which will be switching by backbone switches to communicate data centers, buildings or whatever in layer 2. On the other hand, the Service Provider will have his own VLAN or B-VID and Service Identifiers or I-SID to make services for customers like Internet access, VoIP, replication, backup, etc.

SPB encapsulation

There are two types of SPB, which are SPBV by VLAN and SPBM by MAC-in-MAC. The first one can use the traditional bridging by VLAN and also Provider Bridging in the data plane which are useful for small companies and storage networks. However, SPBM can be deployed in PBB networks where lots of MACs have to be addressed. Nevertheless, both use IS-IS in the control plane and offer layer 2 multipath.


Regards my friends, this is another standard for our shelves.