In the previous post I described the problem STP solves, which is network loops and the devastation they can cause to a network. This post will be fairly brief as I’m going to go over the different iterations of STP over the years, both open standards and Cisco proprietary implementations.
Below are the open standards
- Spanning Tree Protocol – 802.1d
- Rapid Spanning Tree Protocol – 802.1w
- Multiple Spanning Tree Protocol – 802.1s
The difference between these is mainly how quickly they work and how they handle VLANs. The original STP was included in the IEEE 802.1d standard that, from what I can gather, came out around 1990. Like most technology that has been around for a while the main improvements we see are in performance and speed. Same goes for STP. In this original version when a device was plugged in it could take up to 50 seconds before the link became active. In 1990 this wasn’t too big of a deal, however in our fast paced ever connected world that would be a pain. And as that became more the case the standard was updated and STP was improved to have quicker convergence time which is where we get Rapid Spanning Tree Protocol in the 802.1w standard. RSTP/802.1w was released around 2001. This dropped the convergence time down to just a few seconds.
There was still something left to be desired though with both STP and RSTP in that there was one Spanning Tree instance for all VLANs on a network. This is why MSTP/802.1s was developed. This standard allowed you to have multiple STP instances running for a range of VLANs.
Below are Cisco’s proprietary implementations
- PVST/PVST+
- RPVST+
These two technologies are Cisco’s implementations of of the various version of RST. PVST stands for Per VLAN Spanning Tree, and as the name suggests it supports multiple instances of Spanning Tree to be running on a Per VLAN basis. This came out about the same time as the original STP. RPVST+ is Rapid Per VLAN Spanning Tree and just as RSTP improved on STP RPVST improved on PVST’s convergence time. Also, PVST+ and RPVST+ allowed many instances of STP to run, each VLAN could have it’s own instead of a range like in MSTP. That’ll make more sense when we get to how to implement STP in another article.
Thanks for reading,
-Matt
Matt Burks 