CCNA #013: Spanning Tree CCNA Exam Questions: Know the answer? CCNA 200-125 exam

Key Takeaways

[Music] welcome to the vlog so the very first question that you need to know the answer to is which protocol is used to stop loops in a switched environment and the answer is spanning tree this is one of multiple vlogs are discussing spanning tree so spanning tree is a protocol that you run on switches in a layer 2 environment to stop loops from taking place switches will flood broadcast multicast and unknown unicast frames out of all ports except as a port in which it was received this can cause major problems in a network and may result in broadcast storms where the entire network is brought down by broadcasts that are replicated throughout the network so hence spanning tree is a protocol used on switches to stop loops in this topology we've got three switches switch 1 switch 2 and switch 3 so let's start with switch 1 show spanning tree is a spanning tree enabled by default on Cisco switches and the answer is yes on other vendors searches such as those from HPE spanning tree is not enabled by default so be careful if you're using Cisco switches with other vendor switches spanning tree may not be enabled by default but it is fortunately on Cisco switches the command show spanning tree shows us firstly that spanning tree is enabled on the switch it's enabled on VLAN 1 in this example and we are using rapid per VLAN spanning tree even though the output displays rapid STP or or STP this is actually rapid per VLAN spanning tree the priority of the switch is set to 1 this is its address and the switch is currently the root of the spanning tree all ports on the root switch will be forwarding the role of the port is set to designated and I'll explain port roles in a separate video for now let's concentrate on the state of the port engine is three we only have these two ports connected in our topology additional ports are shown here but we'll ignore those ports so ingenious 3 we can update our topology to state 2 that this switch is the route we can state to that this port is forwarding and this port is forwarding once again the show spanning tree command shows us that the supplies to vlan1 at at the moment - that's the only vila and configured on the switch so show VLAN as an example shows us that this is the only ethernet VLAN on the switch these VLANs can be safely ignored because we're not using token ring or FDDI these days you could also use commands such a show spanning tree and specify a VLAN number and use the option summary to see summary information so the switch is known as the root bridge or the root switch for VLAN 1 the reason the keyword bridge is used is that spanning tree was developed for bridges bridges came before switches and hence we talked about a root bridge even though today we actually talking about a root switch no ports are blocking all ports are forwarding on the switch so what about switch t show spanning tree the switch is also running rapid per VLAN spanning tree notice here however we have a cost to get to the root the root switch has a lower priority than the local switch by default in spanning-tree the priority of a switch is three to seven six eight the roots which is determined in a topology based on two criteria firstly lowest priority that's the same on all the switches then lowest MAC address those two combinations make up the bridge ID of a switch the switch with the lowest bridge ID becomes the root but it's based on priority first and then MAC address so because switch one has a lower priority than the local switch the local switch is not to the root if we look at that output on switch one once again what you'll notice is the root ID and the bridge ID are the same so the priority is the same and the MAC address is the same for both the root and the local switch and that's because switch 1 is the root so on switch to the MAC address is different to the MAC address of the root search and in this case so is the priority now the priority on switches is by default 3 to 7 6 8 the extended system ID is the VLAN number that we're looking at so this number 3 to 769 is the combination of the default priority of 3 to 7 6 8 plus the VLAN number which is 1 so the switch has a priority of 3 to 7 6 9 it's not the root we can see that port 1 is the root port of the switch that is the best port to use to get to the root switch that port is forwarding gigabit is 0 1 is known as an alternate port it's an alternative way to get to the root bridge of that port is blocking so this port is blocking on switch to the sport is forwarding and that's what we can see over here now we could also use this command show spanning tree route to see which port is the root port to get back to the roots which essentially the root port is the best port on the switch to use to get to the roots which so gigabit at zero zero is the best port on switch to to get back to the roots which which is switch one what about switch 3 so show spanning tree the switch is not to the root either notice the priority and the MAC address are different than the local switch the cost to get to the roots which is 4 and the root port is gigabit zero zero so on switch 3 this is the root port which is 14 now gigabit is 0 1 is also forwarding it's the designated port in this topology so the port that's blocking in the topology is gigabit zero one now in brief the way spanning-tree determines which ports are blocking is based on the following we firstly elect the route switch so that's switch one all other switches in the topology determine which port is their best port to get to the root bridge that's known as their root port the switches learn about each other because B PDUs are sent between the switches so this port is the root port of switch 2 and this port is the root port of switch 3 we can see that once again by using the command show spanning tree root root port of switch 3 is gigabit to 0/0 on switch 2 shows spanning tree root once again root port is gigabit 0/0 essentially the root port is the best port to use to get to the root switch based on various criteria the first measure is path cost the path cost is a value allocated to each port and they are added together to get the total path cost to the root bridge so the path cost is 4 from switch 2 to switch 1 and the same is true on switch 3 path cost is 4 different versions of spanning tree use different path cost valleys but that's the cost for a gigabit port on these switches so once the switches have determined the root port which is their best port to use to get to the root switch on a per segment basis they choose what's called the designated port the designated port is the best port on that segment to use to get to the root bridge so on this segment the best way to get to the root bridge is via this port on this segment the best way to get to the root bridge is this port so that's the designated port on gigabit is 0 1 on the link between switch 1 and switch 3 we can see that once again by using the show spanning tree command notice both those ports on the root bridge are designated ports they're the best ports to use to get to the root bridge now in this segment this port was the designated port we can see that here gigabit is 0-1 on switch 3 is the designated port and this port is discarding or blocking industry term is discarding Cisco term often used is blocking and we can see that here in the output now the reason why switch 3 was chosen instead of switch 2 to have the designated port is because the MAC address is lower on switch 3 versus switch 2 so switch 2 if you look at the hexadecimal ei is greater in hexadecimal than to one so according to the spanning-tree calculation this is the best port to use on the link between switch three and switch two to get to the root bridge because switch three has a lower MAC address now in spanning tree one of the first things you want to do is manipulate a spanning tree chooses route ports and designated ports and the like that we will discuss that in a separate Vallabh this was a quick introduction to spanning tree and I showed you how ports were selected in the spanning tree environment and how ports were blocked the process very briefly is select the root bridge will route switch based on priority and then MAC address every other switch chooses a root port which is their best port to use to get to the root bridge every segment has a designated port which is the best port on that segment to use to get to the root bridge all other ports in the topology are set to blocking I hope you've enjoyed the video if it's been of benefit to you please like it and please subscribe to my youtube channel I want to wish you all the very best [Music] you [Music]