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Spanning Tree Protocol has evolved significantly since its origins in the 1980s, with IEEE standards and Cisco proprietary versions each advancing convergence speed, VLAN support, and network recovery time. This content covers the major STP variants and examines how Rapid Spanning Tree Protocol improves on the original through simplified port states and faster failover mechanisms.

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About this video

Spanning Tree Protocol traces its origins to 1985, with the first formally defined standard, IEEE 802.1D, established in 1990. While functional, the original protocol was slow to converge — meaning ports took considerable time to transition into a forwarding state after a topology change. This limitation drove the development of subsequent versions aimed at improving speed, flexibility, and scalability across increasingly complex network environments. Cisco responded in parallel by building its own proprietary variants, beginning with Per-VLAN Spanning Tree Protocol (PVST) and later PVST+, which extended 802.1D functionality by allowing each VLAN to maintain its own spanning tree instance, enabling traffic engineering and load balancing across VLANs. IEEE addressed convergence with 802.1W, the Rapid Spanning Tree Protocol standard, and later introduced 802.1S for multiple spanning tree support, while Cisco developed Rapid PVST+ and its own MST implementation to incorporate these advances within a Cisco-proprietary framework. Rapid Spanning Tree Protocol achieves its speed advantage primarily by simplifying the port state model. The original disabled, blocking, and listening states are consolidated into a single discarding state, reducing the time a port spends inactive before it begins forwarding traffic. RSTP also introduces a more precise classification of non-forwarding ports, distinguishing between backup ports — which provide a redundant link to the same network segment, such as a server with multiple connections to a switch — and alternate ports, which represent a secondary path to the root bridge. This distinction allows the network to identify the most appropriate redundant path and activate it significantly faster following a link failure, resulting in quicker overall network recovery.

What you'll learn

What's covered

Spanning Tree Protocol Versions

Aligned to

Cisco CCNA
2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol

Key terms

Spanning Tree Protocol
STP
A network protocol that prevents loops in Ethernet networks by creating a loop-free logical topology.
Virtual LAN
VLAN
A logical grouping of network devices that behave as if they are on the same network regardless of physical location.
802.1D
The original IEEE standard for Spanning Tree Protocol, first defined in 1990, providing loop prevention in Ethernet networks.
Per-VLAN Spanning Tree Plus
PVST+
A Cisco enhancement to STP that runs a separate spanning tree instance for each VLAN, enabling per-VLAN load balancing across redundant uplinks and faster convergence than a single spanning tree. PVST+ uses Cisco's proprietary BPDU encapsulation to carry per-VLAN topology information.
802.1W
The IEEE standard defining Rapid Spanning Tree Protocol, which significantly reduces network convergence time compared to 802.1D.
Rapid Per-VLAN Spanning Tree Plus
Rapid PVST+
A Cisco proprietary spanning tree variant that combines the rapid convergence of RSTP with PVST+'s per-VLAN spanning tree instances, running a separate RSTP process for each VLAN. Rapid PVST+ enables both faster failover and per-VLAN load balancing across redundant switch links.
Multiple Spanning Tree Protocol
MST
An IEEE standard (802.1S) that maps multiple VLANs to a reduced number of spanning tree instances, improving scalability and efficiency.
Alternate Port
A switch port that is not elected as a root port or designated port and is placed in a blocking state to prevent loops, serving as a backup path.
Backup Port
An RSTP port role that provides a redundant connection to the same network segment, typically used when a device has multiple links into the same switch.

Topics

Spanning Tree Protocol Networking Rstp Stp Convergence Vlan Management Cisco Networking

Transcript

Spanning Tree Protocol Versions

Throughout time we have developed spanning tree protocol and made improvements on it, and Cisco has also made additional improvements, so there's some proprietary protocols that just Cisco has.

The original spanning tree algorithm actually dates back to 1985, so it's been around for a while. One of the original spanning tree protocols that was defined was 802.1d, which was in 1990, although it's been developed more since then. So it wasn't something that was stagnant and then stayed that way; it actually developed a little bit more throughout time. But that's the original spanning tree protocol.

Then we get into per VLAN spanning tree protocol, or PVST, or later on it became PVST+. This is a Cisco proprietary, so Cisco developed it. It was based off of that original spanning tree protocol, the 802.1d, but it had some additional functionalities. One of the ones it really had was a per VLAN: you were able to allow different VLANs to have different ways that they would direct traffic. This allowed you to do maybe some sort of load balancing, where you would send some traffic from some VLANs one direction and then another VLAN's traffic another direction.

Then we got into rapid spanning tree protocol. Some of the problems with the original spanning tree protocol was it was very slow to converge, or very slow to actually bring the ports live. So rapid spanning tree protocol was developed to significantly increase the speed that access ports would come back up, and increase the recovery time of the different ports as well. That was defined by 802.1w.

Then Cisco decided that they want to create their own version of this, so they created Rapid PVST+. They took their PVST+ and then they incorporated some of those things that 802.1w had into it and created Rapid PVST+.

And then a more recent one was the multiple spanning tree protocol. This is IEEE's answer to having multiple VLANs, and that was 802.1s. And then Cisco came out with their MST, which is Cisco proprietary, based off of that same one.

So there's a history of both IEEE and Cisco kind of using each other's ideas to come up with some of these protocols. But Cisco would always take whatever track IEEE would come up with and then develop a standard that had some improvements on it, but didn't work with all of the devices, only worked with Cisco devices. It was Cisco proprietary. So those are some of the different versions that you may see out there.

What Rapid Spanning Tree Protocol Changed

What rapid spanning tree protocol did was lessen the time that a port remained in that area where it wasn't really doing much. It combined the disabled, blocking and listening all into one, discarding. So it very much simplified that into discarding, where it still received bridge protocol data units. It still received those, it just didn't throw those away, but it wasn't necessarily forwarding traffic in that time. This drastically reduced the time that spanning tree protocol has to bring up a port live. And then it still has the learning and the forwarding states. So what it does is it just takes the states and reduces the amount of states that these different ports are in. It allows that max age and forward delay, so that 25 seconds, you can recover up to 25 seconds with that by cutting some of that out.

Alternate and Backup Ports

The other thing it did is it gave a little more granularity to a non-designated port. With spanning tree protocol we have this blocked port, or non-designated port, or we also had called it alternate port. Well, rapid spanning tree has what's called a backup port and an alternate port. What this allows is it allows spanning tree protocol to understand what's on the other side of it: is it a switch on the other side, or is it some other server or some other device or resource on the other side? Because if it is an alternate port and then there's a switch on the other side, then I can switch over to that alternate port much quicker. So the idea is, in the event of a failure, then I can turn on that alternate port much quicker.

So maybe you have something like a server that's sitting off here to the side and it has multiple links into your switch. That would be considered a backup port with this. Versus an alternate port would be a secondary route to that root bridge, and so this is an alternate route for other devices to go. So if something were to happen to the original path to that bridge, then it would take that alternate path. So that's the difference between an alternate and a backup port. Adding that designator of alternate versus backup port allows this network to heal much quicker.

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