What Is STP (Spanning Tree Protocol)? And RSTP, MSTP, PVST? [MiniTool Wiki]

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If an active link fails, STP enables a network design to include backup links to provide fault tolerance. It creates a spanning tree that characterizes the relationship of nodes within a network of connected layer-2 bridges. STP also disables those links that are not part of the spanning tree and leave a single active path between any two network nodes.

STP was originally standardized as IEEE (Institute of Electrical and Electronics Engineers) 802.1D. Yet, the functionality of STP (802.1D), RSTP (802.1w), and MSTP (802.1s) has since been incorporated into IEEE 802.1Q-2014.

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RSTP vs STP

To compare between RSTP and STP, first of all, you should know what RSTP is.

What is RSTP? Rapid Spanning Tree Protocol (RSTP), like STP, is also a network protocol. Actually, it is one of the STP standards.

The IEEE introduced RSTP () as 802.1w in2001. By introducing new convergence behaviors and bridge port roles, RSTP offers significantly faster recovery in response to network changes or failures than STP.

RSTP is backward compatible with the standard STP. The standard IEEE 802.1D-2004 incorporates RSTP and obsoletes the original STP.

Also Read: [Library] Everything About RTSP: Real-Time Streaming Protocol

MSTP vs RSTP

Similarly, to learn the differences between MSTP and RSTP, it is recommended to learn the definition of MSTP firstly.

Multiple Spanning Tree Protocol (MSTP) is also a standard of STP network protocol to map one or more VLANs. Together with the algorithm, it provides full and simple connectivity assigned to any given virtual LAN (VLAN) throughout a bridged local area network.

MSTP makes use of Bridge Protocol Data Units (BPDUs) to exchange information between spanning tree compatible devices. By selecting active and blocked paths, it prevents loops in each Multiple Spanning Tree Instances (MSTI) and in the Common and Internal Spanning Tree (CIST). Also, in STP, this is done without manually enabling backup links and getting rid of bridge loops danger.

Besides, MSTP allows packets/frames assigned to different VLANs to follow separate paths, each based on an independent MSTI, within MST Regions composed of LANs or MST Bridges. Those Bridges, Regions, and LANs are connected into a single Common Spanning Tree (CST).

CSTs are backward compatible with the STP and RSTP. An MST that has a single VLAN assigned to it is an IST (Internet Spanning Tree). Unlike some proprietary per-VLAN spanning tree implementations, MSTP includes all of its spanning tree information in a single BPDU format. This not only reduces the number of BPDUs but also ensures backward compatibility with RSTP and typical STP (in effect).

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Besides, MSTP is completely compatible with RSTP bridges for an MSTP BPDU can be interpreted by an RSTP bridge as an RSTP BPDU. Also, this causes any RSTP bridges outside of an MSTP region to see the region as a single RSTP bridge.

To further facilitate this view of an MST region as a single RSTP bridge, the MSTP protocol uses a variable known as remaining hops as a time to live counter instead of the message age timer of RSTP.

Ports at the edge of an MST region connected to either an RSTP or STP bridge or an endpoint are known as boundary ports. While in RSTP, when connected to endpoints, these ports can be configured as edge ports to facilitate rapid changes to the forwarding state.

MSTP was originally defined in IEEE 802.1s-2002 and later merged into IEEE 802.1Q-2005. It defines an extension to RSTP to further develop the usefulness of virtual LANs.

Related Article: RTMP (Real Time Messaging Protocol): Definition/Variations/Apps

PVST vs RSTP

Still, let’s learn what does PVST stand for first.

Per-VLAN Spanning Tree (PVST) is a kind of proprietary network protocol developed, implemented, and published by CISCO. It uses its own proprietary ISL (Inter-Switch Link) for VLAN encapsulation while PVST+ uses 802.1Q VLAN encapsulation.

Before the IEEE published a Spanning Tree Protocol standard for VLANs, many vendors who sold VLAN capable switches like Cisco developed their own Spanning Tree Protocol versions that were VLAN capable. Cisco also published a proprietary version of RSTP called Rapid Per-VLAN Spanning Tree (RPVST). RPVST creates a spanning tree for each VLAN like PVST. 

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