CCNA #019: Spanning Tree CCNA Exam Questions: Root Bridge, Root Port and more: CCNA 200-125 exam

Key Takeaways

[Music] [Music] in this vlog we're going to discuss a spanning tree have a look at this topology which which is the root of this topology and why okay so how is the spanning tree root of selected it's based on the bridge ID which consists of the priority and MAC address of the switch and if you want to get more technical it's the priority plus extended system ID plus the MAC address so having a look at the bridge IDs of these switches let's start with the priorities so for zero nine seven four zero nine seven three two seven six nine three two seven six nine so the two switches with the lowest priority of switch one and switch two now we can't use the priority as a tiebreaker so we need to look at to the MAC addresses of the switches so first for hex values are zero zero zero four stay the same then we have one 8d two and on this side we have one eight one C so I would say that the switch is the root because it has the lowest priority and MAC address so on switch to shows spanning tree notice the roots which has a priority of four zero nine seven MAC address is this that is the priority and MAC address of the local bridge or local switch and that is the values for switch two so switch two is the root in this topology now in the old days the switches would have a priority of 4 0 9 6 as an example but these days the extended system ID is used in combination with the priority so the priority for VLAN 1 is 4 0 9 7 on the switch which is a combination of the priority plus the extended system ID have a look at my CCNA course for more details of that we'll have a look online for extended system IDs so this switch is the root of the topology so here's another question in the spanning tree calculation what is determined next once you have the roots which of the topology what is selected next and the answer is root ports every switch in the topology needs to choose a route port which is its best port to get to the root bridge so as an example on switch for gigabit zero zero is going to be the root port because that's the best port to use to get to the root switch or root bridge we can confirm that by typing show spanning tree and VLAN and the VLAN number which is VLAN 1 in this case root as you can see the root port on the local switch to get to the root bridge is gigabit 0/0 the path cost to get to the root bridge is 4 root pods are determined based on four criteria lowest path cost if that's the same then it's the lowest neighbor bridge ID if that's the same then it's the lowest port priority if that's the same then it's the lowest port ID so which port on switch 1 will be the root port which port do you think will be the root port on switch one is at gigabit zero one or gigabit zero zero the answer is most likely gigabit zero zero assuming that we've got default values typically in the exam they will let you know if they've changed something so show spanning tree on switch one shows us that gigabit is zero zero is the root port gigabit zero one is an alternate port we could also use the command once again show spanning tree VLAN one root we can see that gigabit to zero zero is the root port of switch 1 now the reason why gigabit to zero zero is chosen before gigabit to zero one is because the priorities of the ports are the same but notice the port number of gigabit to zero zero is 1 and the port number of gigabit to zero 1 is 2 so gigabit to zero zero has a lower port number so that is selected as the root port so gigabit zero zero will be the root port of switch 1 now what about switch 3 which port is its root port path costs are going to be the same let's assume that all these links are gigabit links path costs haven't been changed so we can't use a path cost to determine the best path to the root bridge we need to use something else and that's a lowest neighbor bridge ID so look at switch one bridge ID look at search for sprig ID which one is the lowest it's going to be such one so the root port on switch 3 is gigabit as 0 0 let's confirm that though so on switch three shows spanning tree VLAN 1 root port root port is gigabit 0/0 so this is going to be the root port of switch 3 so once we've determined with the root switches of the topology or root bridge of the topology is we then select root ports which we've done now what gets selected next in the spanning tree calculation what's the next option so the next step is to choose designated ports designated ports are chosen per segment so you need to look on every segment and determine which port is the best port to use to get to the root bridge so this is going to be the best port to use to get to the root bridge on switch 2 I would say and I would say the same on these ports generally all ports on a root to switch are designated ports except in some cases as an example if you loop cables on the route switch so show spanning tree gigabit 0 0 0 1 and 0 2 are designated ports they are forwarding the switch does have other ports shown in the output here that the only ones connected in our topology or gigabit 0 to 0 1 and 0 0 so those are the best ports on the segment to use to get to the root bridge so on this segment which port is the designated port put your mouse in the middle of this cable and work out is this a quicker path or is this a quick upon the pot costs of all these links are the same so this is one and a half links this is half one and a half two and a half so it's going to be quicker to go via this port so this will be the ignited port on switch for to confirm that show spanning-tree on switch for Lotus gigabit zero one is the designated port it's the best port to use to get to the root bridge on the segment what about the segment we could go this way or we could go this way so the quickest way is going to be via that port on switch one let's confirm that shows spanning tree gigabit to 0-2 is a designated port it's forwarding so we've now worked out to the route bridges we've worked out to the route ports and we've worked out the designated ports all other ports in the topology become blocking or discarding ports so blocking blocking let's prove that on stage one again gigabit zero one is what's called an alternate port it's blocking notice the state is blocking so this is an alternate port and it's blocking on switch three show spanning tree this is an alternate port and it's blocking so gigabit zero one is also blocking as an alternate port so we've successfully determined who the route switches or route bridge which puts our route ports which ports are designated ports and which ports are blocking now remember that physically the topology looks as follows but logically this link is disconnected it's not sending user traffic at all and logically this link is disconnected it's also not sending user traffic so for PC on switch three were sending traffic to a PC on switch for it would have to traverse this link this link and this link to get there in my CCNA course I talk about a campus topology and how to optimize the campus topology so have a look at the course for more information or have a look online for ways to optimize spanning tree I'll also talk about some of the options and the vlogs so thank you for watching if you've enjoyed this video please like it and please subscribe I wish you all the very best [Music]