Redundancy in communication networks is critical in substation automation, processing, and manufacturing application. The Spanning Tree Protocol (STP), which was originally designed to address the problem of network loops, quickly became popular for providing path redundancy in case of network faults. Redundancy kicks in by re-configuring the network once the fault is detected and in case of STP the time taken is in the order of tens of seconds. Rapid Spanning Tree Protocol reduces the reconfiguration time to few seconds.
- RSTP is a Layer-2 protocol that is implemented in the bridges and switches.
- A root switch/bridge (“node”) is identified which serves as the central point in the network.
- The network ports in the remaining switches identify the shortest path to the root node. Packets are then forwarded via these shortest paths.
- The link causing a closed loop is identified and blocked. The blocked port can still receive packets, but it does not forward them.
- Bridge Protocol Data Unit (BPDU) frames are used to configure the spanning tree network and communicating a topology change request when a fault is detected.
- In case of topology change in the network, the blocked port is opened to provide an alternative path for communication.
We have advanced level expertise with the design and implementation of RSTP on the Texas Instruments Sitara platform with Linux/RTOS running on the ARM core and the firmware running on the Programmable Real-time Unit (PRU) core within the Industrial Communication Sub-System. Discarding or forwarding of packets based on the port state on the receive side has been offloaded to the firmware to improve the OS performance on the ARM host. The implementation has been tested rigorously under heavy traffic conditions in conjunction with the PTP protocol. Further enhancements in the performance of the driver and the firmware is currently work in progress.