Spanning tree: everyone's favorite protocol! Thousands of pages have already been written about spanning tree, so I've decided to take a different approach.
I find it helpful to visualize protocol elections and traffic flow in order to better understand protocol behavior, so I created a visualization illustrating the initial spanning convergence process. This visualization only addresses the
initial root bridge election and STP convergence process, for example, if all switches were to boot at the same time. This does not address converging a new STP topology after a topology change.
The visualization below is basically an embedded slideshow that can be advanced by clicking on the image. There are notes for each slide that briefly explain each step of the convergence process. The numbers on each side of the links between switches represent the port number of each uplink.
Here are the basic steps of the initial STP convergence process:
- Elect the root bridge.
- Determine the root ports.
- Determine the designated ports.
- Remaining ports are blocking ports.
Click the image to advance the visualization.
Tap here if you are on a mobile device.
As you can see, the resulting topology (the logical forwarding topology that is created after STP blocks redundant links) looks something like an upside-down tree.
Here is an example of of what this tree might look like when
redundant links are blocked by STP in a larger L2 topology (although hopefully your network looks nothing like this). The links shown in black can be used, because each port is in the forwarding state. The links shown in red cannot be used, because one of the ports is in the blocking state. Reducing the topology to a tree of forwarding links is how spanning tree maintains a loop free L2 topology.
Once again, this is not meant to be a complete description of STP, but rather a visualization and basic description of the initial convergence process.
Feel free to leave questions or comments below.
