Spanning Tree Protocol (802.1d) has and remains one of the pillars of modern switched networks, quietly policing Switching Loops, and preventing Broadcast Storms in our Ethernet Networks. STP works by sending messages (called BPDUs) between the switches (called Bridges) electing one bridge to be a central switch or rendezvous point for the network (called a Root Bridge). All the other switches will then use the BPDUs to calculate the best path to get from them, to the central switch. STP will then disable all but these best paths between the switches, leaving only the single route from each switch to the Root Bridge. STP remains running in the background, ever watchful for any link coming up or going down. If a link goes down it will rerun the above process and bring up one of the backup (redundant) links. If a link comes up it will first check to see if a Switch has been added to the network and if so, will this switch become the new Root Bridge? It will then rerun the above process enabling/disabling the links as required. The problem with STP is that all this takes time to elect Root Bridges, and calculate best paths. Modern networks require a much faster recovery time than the thirty to fifty seconds offered by STP, and it is this that has driven the introduction of newer flavours of Spanning Tree.
Rapid Spanning Tree Protocol (RSTP 802.1w) then arrived on the scene, slashing recovery times down to much more respectable one to five seconds. It achieves this dramatic improvement by adding a few simple improvements to the Spanning Tree process. Firstly instead of calculating the best path between switches and forgetting the others, it remembers these backup links. By keeping them in reserve, when a link goes down, it is able to bring up the backups straight away without having to recalculate the whole network topology. These new backup links are the alternative and backup ports. Other improvements include the introduction of ‘Port Fast’, where if an end user device like a PC connects to the switch, STP moves the port from down to up without waiting for timers to expire. It also includes the idea of ‘BPDU Guard’ where it will still protect the network, if someone connects a switch to a ‘Port Fast’ port.
We are now using the final flavour of Spanning Tree (802.1d 2004). This is somewhat confusing as it is much the same as Rapid STP, and in fact incorporates 802.1w protocol as part of the standard. However, it also requires that 802.1d 2004 is fully compatible with both Multiple Spanning Tree (MST) and the original version of STP (802.1d). MST allows (if required) a separate Spanning Tree topology to be calculated for different VLAN, so some links be blocked for some VLAN and not for others. This gives the benefit of simple load balancing, and further reinforcing that each VLAN acts like a separate network. 802.1d 2004 also includes features similar to BPDU guard and Port Fast, in a fully industry standard version of Spanning Tree. The final piece in the puzzle is that 802.1d 2004 further refines the Spanning Tree algorithm, to improve network recovery to a fantastic thirty to fifty milliseconds.