CCNA Quiz: Spanning Tree Explained with Wireshark Captures: Which ports are blocked? CCNA | CCNP

[Music] can you answer this quiz question which ports in this topology of blocking we've got switch 1 switch 2 and switch 3 the switches have these priorities these are the MAC addresses off of the switches I've pre-configured - these switches in genus 3 and I'll show you the answers in a moment but can you work this out notice the port numbers and notice the priorities notice the MAC addresses which ports are blocking in this topology what do you think pause the video now if you want to work it out otherwise I'm going to now show you the answer ok so the first decision in a spanning tree topology like this is to determine the root switch which a switch is the route to switch that's based on the bridge ID which consists of the bridge priority and the bridge MAC address priority takes precedence so we look at the priorities and we only use the MAC addresses as a tiebreaker a lowest number wins this is a tip to remember generally and there's always exceptions but generally on switches the lowest number ones and on routers the highest number ones so as an example with HS or P the highest priority wins but on spanning tree on switches the lowest priority ones so again a tip to remember on routers generally highest number wins on switches generally or lowest in number wins okay so I would say that the switch is the root switch so switch 1 is going to be the root because it has the lowest priority in the stop ology again on running this topology engine is 3 so I'll show you the answer in a moment but for now let's work it out that is the root switch of the topology the next decision is every switch that is not the root switch needs to choose it's a root port that's the best port to use to get to the root switch based on a path cost these are gigabit links path cost by default here is for from switch to s point of view it has two paths to get to switch one of equal cost so we can't use path cost as the determining factor next decision is look at the neighbor bridge ID lowest number wins in this case we can't use that as a determining factor because both those ports are connected to such one so we can't use neighbor bridge ID next decision is based on priority port priority that's based on the side so when the BPD use or bridge protocol data units are forwarded to switch to the priority and port number allows us to determine which port is going to be the route port so I'm going to start a wire shock capture on that port here's the output of that capture notice we have a spanning tree message being sent into the network it's automatically converted the name of the switch based on the vendor code I just use some random numbers here I could have used Cisco as an example rather than 0 0 to 0 but notice inside of the message we have spanning tree root identifier is this 4 0 9 6 path cost is 0 to the root notice the one that is the system ID extension in other words this is a bpdu for VLAN 1 so root bridge priority root bridge system ID and the MAC address of the root cost is 0 to get to the root notice the bridge information is exactly the same because of this is the root switch notice the port dennah fire notice that number for port let's do a capture on the other link so here's gigabit one zero two gigabit zero 1 and here's gigabit zero three two gigabit zero zero and if we look in the message we can see that the port identifier over here notice this number is smaller than this number so in the bpdu we actually see the port information advertised to the other switch so the switch knows that the BPD use coming from gigabit zero three have a lower port number than the BPD use coming from gigabit one zero so again just to make the point gigabit one zero port identify as this gigabit zero three port identifier is this and if we look at to the console of that switch show spanning tree we can see the port numbers notice gigabit one zero priority is 128 port number is five gigabit zero three priority is 128 port number is four so in the wireshark capture notice this is gigabit one zero gigabit one zero has a port number of five notice we can see it over here port number of five and gigabit zero three has a port number of 8004 notice gigabit is 0-3 there we can see the port number now what I'll do just to make a point J is I'll change the priority of let's say gigabit zero three so I'll say spanning tree port priority 64 and let's see what the BPD use look like so this is on gigabit zero three here's the wide shot capture I'll scroll down right to the end so here are the latest be BTUs notice the port identifier has been reduced so the combination of the priority and port number have reduced this value I'll set it back to the default of 128 scroll all the way down and notice it's changed again 8th 2004 whereas previously it was four thousand and four so again four thousand and four now it's back to eight thousand and four so at the moment the priorities are set to the default 128 on all the ports so based on that information this port is going to be the root port because and this interface has a lower port number priorities are the same port number is lower we can see that once again on switch one show spanning tree this interface has a lower port number than this interface so that'll be the root port and to prove that let's have a look at the console it's a show spanning tree notice the root port is gigabit as zero zero which is correct per our calculations this is the root port okay so what about the side assume that are the priorities are the same 128 on both these ports which port is going to be the root port on switch 3 so again looking at path cost path costs are the same to get to the root bridge if we have a look at the neighbor bridge ID it's the same on both these ports so we can't use neighbor bridge ID as the determining factor if we look at priorities the they are the same we can see that again on switch to notice the priorities are the same then we need to base the decision on port number this port number is lower than this port number I'll stop these why shot captures and I'll prove it again by doing a wire shot capture on gigabit is 0-2 and and gigabit 0-3 so on gigabit zero 2 if we have a look at the spanning tree protocol notice root identify is this that's which one the local switch is this 3 2 7 6 8 is a priority this is the MAC address and that's the switch if we have a look at the port identifier notice it's 0 x8 0 0 3 that's gigabit 0 to notice interface number here priorities 128 if we look at gigabit 2 0 3 there's the bridge ID there's the route there's the port identifier a 2 0 0 4 so this port has an identifier for which we can see over here notice poor identifier is for soloist port number will win I would say that this will be the root port in the topology on switch 3 so let's have a look at switch 3 and confirm that so here's switch 3 show spanning tree notice root port is gigabit as 0 1 not gigabit 0 0 so this is the root port in the topology now once we've got root ports we need to determine designated ports so again in spanning tree with per VLAN spanning tree there's one route per VLAN for the entire layer 2 topology once we've chosen the routes which we need to choose route ports any switch that's not the root switch has a root port which is its best port to use to get to the root bridge or roots which we've chosen that now so now on a per segment basis we need to choose designated ports now you can't get closer to the root switch than the root switch itself so all interface is connected to the routes which I immediately set it to designated ports on this link here what's the quickest way to the root switch is it to go round here or was it simply to go left I would say it's quicker to go left so that's a fairly easy decision for both of these ports it's going to be quicker to go left than it is to go right if you are on this segment or if you are on the segment so those are designated ports all other ports on are blocked so this becomes a blocking port I'll change the color of this text to red to indicate blocking so that is a blocking port and this is a blocking port so there you go we've worked it out let's confirm that we've done it right on switch to show spanning tree notice gigabit is 0 1 is blocking it's an alternate port so we've worked that out correctly notice gigabit is 0 2 and 0 3 are designated ports root port is gigabit 0 0 and then on switch 3 show spanning tree we can see that gigabit is 0 0 is blocking for our calculations 0 1 is the root port now in genus 3 the switches see other interfaces are up there are actually down but because this is a virtual environment they are shown as up that may change in later versions of genus 3 but you can basically ignore all these other interfaces because they not connected in this topology so there you go how did you do did you find this useful if you did please like this video and please if you don't mind subscribe to my youtube channel that really does help me I'm David bumble wanna wish you all the very best [Music]

Channel: David Bombal

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Keywords: CCNA, ccna, gns3, STP, spanning tree, spanning-tree, root switch, root ports, designated ports, blocking ports, switching, briding, CCNP, CCNA Training, CCNP Training, CCENT, ICND1, INE, CCNA Study, CCNP Study, cisco ccna, cisco, ccie, icnd1 exam, infrastructure, icnd1, ccna training, ccnp, cisco certification, david bombal, GNS3, gns3 labs, gns3 tutorial, how does spanning tree work, stp, spanning tree protocol, ccna routing and switching, ccna exam, ccna lab, gns3 ccna

Id: YG8MmTTjikA

Length: 13min 32sec (812 seconds)

Published: Tue Mar 05 2019