Spanning Tree Protocols – STP, RSTP, MSTP

STP-is the short form for Spanning Tree Protocol & RSTP (Rapid Spanning Tree Protocol), MSTP (Multiple Spanning Tree Protocol) are all advanced/ improved implementations of STP. In this article, we will try to understand the basic concepts of Spanning Tree Protocols and their implementation.

What is Spanning Tree Protocol (STP) and why is it required?
Spanning Tree Protocol, and its improved versions, are required mainly for two reasons – To prevent network loops (due to multiple paths to the same destination) & to introduce redundancy in the link connections (if one link fails, the data is still routed through a different link/route).

Network loops sometimes happen inadvertently, especially in large networks. When there are two or more paths to the same destination, there is a danger of broadcast packets getting in to an infinite loop and hence causing congestion in the network. So, STP is used to identify the best path to the destination, and block all other paths (links). The blocked links are not always discarded. They are connected and kept inactive by STP, so that, when the best path fails – the next best path can be achieved by activating the blocked links.

RSTP – Rapid Spanning Tree Protocol:
The above processes are common to STP/RSTP & MSTP. With STP, the detection and reconfiguration of network topology changes (when a cable is cut/ new switch is added) takes some time – like 30-50 seconds. Since, a lot of critical / time sensitive applications are running on the LAN, this inactive period may not be acceptable. So, Rapid Spanning Tree Protocol (RSTP) was conceived to overcome this problem (RSTP takes 5-6 seconds to update and re-configure the new network topology/ routes).

In RSTP, link status of each port are monitored pro-actively (instead of waiting for the BPDU messages) to detect network topology changes. RSTP is backward compatible with STP switches.



The Switch ports that participate in RSTP have three states – Discarding (Does not accept/ forward any data but listens to BPDU messages), Learning (Once the network topology change is detected/ activation request comes via the BPDU message and filtering/ forwarding table creation is initiated) & Forwarding (RTSP ports start accepting and forwarding data packets/ frames).

MSTP – Multiple Spanning Tree Protocol:
MSTP (Multiple Spanning Tree Protocol) can map a group of VLAN’s into a single Multiple Spanning Tree instance (MSTI). Which means, the Spanning Tree Protocol is applied separately for a set of VLAN’s instead of the whole network. Different root switches and different STP parameters can be individually configured for each MSTI. So, one link can be active for one MSTI and the other link active for the second MSTI. This enables some degree of load-balancing and generally two MSTI’s are used in the network for easier implementation.