if you had two switches and you connected them together with an ethernet cable and then you connected another ethernet cable to those switches you would create a loop on the network this means an Ethernet frame would loop around that particular connection until you disconnected one of those cables there's no mechanism at the MAC address level to count how many times you've seen a frame so as these frames are going around this Loop you're adding more and more frames to the network all the time those are continuing to go around this Loop and eventually you'll overwhelm the capacities of your switches this can happen relatively quickly once you create a loop in a network it might be a number of seconds until people start calling the help desk stating that the network is down fortunately this is relatively easy to resolve you simply go back to the switch where you plugged in that cable you disconnect the cable which removes the loop from the network and now everything is back up and running but of course we're not always standing in the data center ready to plug or unplug a cable from a switch we need some type of automated method to recognize a loop on the network and to prevent those Loops from occurring fortunately we have a standard that's able to recognize and prevent loops on a network this is the i e 802.1d standard one that we call the spanning tree protocol spanning tree can configure interfaces on our switch in a number of different ways it may configure the switch switch port to be blocking which means it's not going to forward any traffic this is commonly used to prevent loops on a network if we were to plug in that second cable to our switch spanning tree would recognize that a loop was being created and it would cause one of those interfaces to be in a blocking State the listening phase is used by spanning tree to clear out existing tables and restart the process it's going to go through a learning process where it listens to understand where all of the other switches might be and if they're might be any loops on the network already running and then it will configure an interface to be forwarding which means traffic can be transmitted through that interface or will configure an interface to be blocking we might also have interfaces that are administratively disabled those obviously would not participate in this spanning tree process here's a network that is already running spanning tree and there are a number of switches they're identified as Bridge one Bridge six Bridge five and so on you can see there are five of them in this particular design and you can see that many of these switches are connected to each other and if we didn't have spanning tree running there would be a loop on this network already but on this network we already have spanning tree that's operational and it is configured a number of different interfaces there are three different types that you can see on the screen one is the root port or RP this is the interface on a switch that is connected ultimately to the single root bridge on the netor Network there's also a designated Port which would be any other Port that can forward traffic but is not the port that is connected directly to the root bridge and there are blocked ports these are ports where spanning tree has disabled that Port from sending or receiving any traffic you can see on this network that bridge 21 has a blocked port and Bridge 11 has a blocked port and those have been blocked so that there is not a loop on the network if we were communicating from Bridge 11 you can see there is a single root port and you can follow that all the way back with those root ports to the final rot Bridge the designated ports are also available to be able to send and receive traffic in this scenario let's say that we're on network a and we've been communicating with a device on network M you can see that we can communicate through Bridge 6 which has a designated port and a root Port that allows us access to network M but what if there is a problem on this network and we lose connectivity between Network a and Bridge 6 now we need some other way to communicate to network M but as you can see there is a blocked port on Bridge 11 so we would not be able to communicate through that connection fortunately spanning tree recognizes that we have this disconnection and it begins clearing out the existing configuration and relearning the topology of the existing Network spanning tree has changed Bridge five so that the root Port is now on the other side and it has removed the block port on Bridge 11 allowing us onet Network a to be able to now communicate all the way through the network back to network m without having another loop on the network on many networks we do talk about enabling spanning tree and making sure that our spanning tree configuration is turned on inside of our switches but in reality these days we're probably running a newer version of spanning tree called the rapid spanning tree protocol the standard is the 802.1w standard in the original spanning tree protocol it might take 30 to 50 seconds for the convergence process to occur and on today's networks that is a very long time to wait without any type of connectivity with rapid spanning tree protocol this convergence process goes down to about 6 seconds which means anytime There's A disruption on the network we can be back up and running relatively quickly fortunately this rapid spanning tree protocol is backwards compatible with the original spanning tree so if you have a mixed set of devices on your network that support one or the other standard you can run them all at the same time fortunately rapid spanning tree protocol works very similar to the original spanning tree configuration it simply operates much faster there are some shortcuts and other configuration settings within the rapid spanning tree protocol but it has such familiarity that if you understand spanning tree protocol will be very easy for you to understand the details of Rapid spanning tree protocol