Spanning Tree Protocol | CCNA - Explained

hello everyone in this video we're going to be talking about the spanning tree protocol now when network devices want to communicate with other network devices they first need to know the MAC address of those devices and the way they find the MAC address of a device that they want to talk to is by broadcasting a signal out on the network asking for that device's Mac address and then once the MAC address is known communication can take place so for example if computer a wanted to communicate with another computer on this network it'll send out a unit of data called a broadcast frame and then once the broadcast reaches the switch will forward that broadcast to every device that's connected to it now in a typical Network you would have a switch with computers connected to that switch forming a local area network however in some cases Network administrators may want to add redundancy to their Network in case of a switch or a cable failure so instead of having one switch they may use multiple switches so for example this network is using three switches on this network so that means that this computer here has two options it can take to communicate with this computer over here it can communicate using this path here or if this link goes down it can use this path instead and vice versa but the problem with having a setup like this is that it could create a problem called broadcast Loops so for example as I stated before whenever a computer wants to communicate with another computer it first has to send a broadcast frame out on the network to find the computer it wants to talk to so if computer a computer C computer a will send out a broadcast frame to the switch it's connected to which is switch one and remember switches will always forward broadcasts to every device that's connected to it so switch one will forward the broadcast to computer B switch 2 and switch 3 and then when switch 2 receives the broadcast it will forward the broadcast to computer C and computer D but it's also going to forward it to switch 3 and then when switch 3 receives the broadcast it'll forward the broadcast to switch one and switch one will for the broadcast again to switch to and then this will continue on in a NeverEnding Loop and the same thing happens in the other direction when switch 3 receives a broadcast from switch one it'll forward the broadcast to switch 2 and then to these computers again and then switch 2 will forward the broadcast to switch one and then it's rinse and repeat the whole network will be caught up in a NeverEnding Loop of broadcasts which is known as a broadcast storm and when this happens the network can't do anything because of a constant broadcast and the entire network will come to a grinding halt so this is why the spanning tree protocol was developed it was designed to prevent broadcast Loops when multiple switches are used on a network so the next question is well how does STP prevent this well the short answer is that it does this by blocking certain ports on the switches and so the next question is how does it determine which port or ports to block well the first thing that STP does is to determine which switch will be the root Bridge will be considered the most important switch and the way it determines this is by having all the switches talk to each other and they do this by sending out messages called bpdus or Bridge protocol data units out on the network these units contain information called the B ID or Bridge ID the bridge ID consists of the switch's priority number plus the VLAN number along with its Mac address and STP uses the bridge ID to determine which switch will be the root bridge and the switch with the lowest Bridge ID value will be considered as Superior and will be elected as the root Bridge so for example by default each switch will have a priority value of 327 68 and let's say that all the switches are on VLAN 1 so we will add the one to the priority value which will now equal 32769 but since all the switches on this network have the same priority value that means it's a tie however the tiebreaker will be determined by the MAC address so whichever switch has the lowest Mac address will be elected as the root Bridge so we see that switch 3 has the low to MAC address which means

that it'll be elected as the root bridge and the ports on the root Bridge are labeled designated ports are ports that lead away from the root bridge now the next step is to determine the root ports are ports on the non-root switches so these switches here that forward data to the root Bridge root ports are elected by What's called the lowest path cost which means that the port on the switch that's the fastest link to the root Bridge the path cost is calculated in the following chart if the link speed is 1 gbits per second the cost is equal to 4 if the link speed is 100 megabits per second the cost is equal to 19 and if the link speed is 10 megabits per second the cost is equal to 100 so let's say that all the links between the switches are 100 megabits per second and if we look at our chart 100 megabits will have a cost of 19 so all three links will have a path cost of 19 so starting with switch one it has two paths to the root bridge this one here or this longer one here so this path here has a cost of 19 but plus another 19 which equals 38 so since this path here has a lower cost this port on this switch will be the root Port because as I stated before whichever Port has the lowest cost path to the root bridge will be the root port and the same thing goes with switch to this path has a cost of 19 and this other 38 so this path is lower so this port will be the root Port now just to give you another example let's say that these two links here are 1 gbits per second each and if we look at our chart that would give them a cost of four each so going back to switch one if we add up the cost of this path it would be 4 + 4 which equals 8 and the other path is still equal to 19 so since 8 is lower than 19 this port would be the root Port so this is how root ports are determined so let's go ahead and put these links back to 100 megabits each and continue on with the lesson so now that we have our root ports figured out it's time to determine which of these last two remaining ports are going to be blocked to prevent broadcast loops and the way to determine which Port will remain open and which one will be blocked is again based on the switch's bridge ID and since the lower Bridge ID wins the port on switch 2 will remain open and be laid AED a designated Port but the port on switch one because it has a higher Bridge ID its Port will be blocked so by blocking this port it will shut down this link which will prevent any broadcast Loops so STP has to go through all these steps to find out which port or ports to block but if any of these other links or devices were to go down STP will reactivate this port and bring this link back up the spanning tree protocol was developed in 1985 as the i e 802. 1d standard and in 2001 rstp or rapid spanning tree protocol was developed which was label the i e 802. 1w standard rstp was an improvement to STP by significantly adapting faster to a failure or to a change in the network so where STP contain take 30 to 50 seconds to adapt rstp can do it in under 6 seconds and it is also Backward Compatible to STP guys if you want to do your own little experiment you can do this to create your own broadcast storm just to get an idea of what happens so you just need a standard inexpensive switch with no Loop blocking features plug in your computers into that switch and then get a network cable and then plug both ends of the same cable into the switch and see what happens so what's going to happen is when these computers send their broadcast frames to the switch is going to endlessly Loop the broadcast and as it does this you will see the LED lights on the switch rapidly flashing in sync which means that this network is in a broadcast storm and the computers won't be able to do anything they exchange data between them or if they are connected to the internet they won't be able to

access anything on the internet so if you try to bring up a web page nothing will happen so this is actually a simple way to bring down a local network but as soon as you remove the cable everything will go back to normal so guys I hope you learned something from this lesson but speaking of learning I would like to thank the sponsor of this video which is brilliant is an online learning center where you learn by doing with thousand thousands of interactive lessons in math data analysis programming and AI it's a platform where you can learn different skills that are in demand today but what makes brilliant different is that it makes learning fun and effective because you're not just watching a lecture or reading text but instead you're involved in doing the exercises yourself which makes complex ideas easy to grasp so for example are you interested in programming brilliant has a growing number of programming courses that are a great way to build foundations and learn real world applications such as getting familiar with python where you can start building programs on day one with a built-in drag and drop editor you can also learn essential coding elements from loops and variables to nesting and conditionals and you can also learn to develop your mind to think like a programmer and begin writing complex programs to build games and apps and right now you can try everything that brilliant has to offer for free for a full 30 Days by going to brilliant. org slower or click the link in the description or just scan the QR code on the screen and you'll also get 20% off an annual premium subscription