Cisco CCNA 200-301 – STP – Spanning Tree Protocol Part 3

6. Spanning Tree Versions

This lecture you’ll learn about the different spanning fee versions. There have been a few different versions over time which have improved on the previous versions. Now when I was first learning about this from other sources it was super confusing but there is actually a simple way to explain it which is by by breaking it down into the Open Standards and also into the Cisco proprietary versions. So that’s what I’m going to do for you here. Starting off with the Open standards the first original implementation of spanning three was eight two 1D that uses one spanning three for all of the different VLANs in the land.

So just one instance for everything that was improved with version eight two one W which is rapid spanning tree. So that improved spanning tree by significantly improving the convergence time. With 802 1D it can take up to 50 seconds for an interface to make sure that there’s no loops there and transition into the forwarding state with rapid spanning three that gets down to typically a few seconds. Rapid spanning three also uses one spanning three instance for all VLANs in the land.

The latest of the industry standards is 802 one s which is multiple spanning tree that enables grouping and mapping VLANs into different spanning three instances which allows you to do load balancing. So to summarize those 802 original implementation it’s got very slow convergence time and it doesn’t support any load balancing. 802 one W came out after that which improved the convergence time but also did not support load balancing.

The latest 1802 one s builds on rapid spanning tree by keeping the improved convergence time and it also enables load balancing as well. So let’s have a look and see how the load balancing works. The axis layer switches in our example here have got PCs which are attached in multiple different VLANs. We’re going to make CD one the core distribution switch one the root bridge for VLANs ten to 19. So now traffic for those VLANs is going to be forwarded on the link to CD one and blocked on the link to CD two.

So we’re looking at it from the point of view of our access layer switch access three here. So when we configure this traffic for VLANs ten to 19 are going to go up the uplink to CD one. CD two is going to be made the root bridge for VLANs 20 to 29. So traffic for those VLANs is going to go up the link to CD two and it will be blocked on CD one. So half my traffic goes on the uplink to CD one. Half the traffic goes on the uplink to CD two. If either one of those uplinks fails then all traffic will fail over to using the one link. And with MSTP multiple spanning tree we’re going to have two spanning three instances running one for each group of VLANs. So that’s how it allows us to do load balancing and those were the Open Standards. Next up, let’s look at the Cisco proprietary versions.

First one is PVST plus. This came out around the same time as 802 1D, but it included Cisco’s enhancements. The main enhancement is it uses a separate spanning three instance for every VLAN.

So per VLAN spanning three plus allows you to do load balancing the same as multiple spanning three does. But because this came out around the same time as the original 802 1D, it’s got the same issues with having a very long convergence time. PBST plus is the default on Cisco switches. So the default on Cisco switches, you’ve got a separate spanning three instance for every single VLAN, and it’s got slow convergence time.

The next Cisco version was Rapid per VLAN spanning three plus. This came out at around the same time as 802 one W, which if you remember from the Open Standards, was the second implementation which had a faster convergence time. So rapid PVST plus also significantly improves the convergence time over PVST plus. Like PVST Plus, it uses a separate spanning three instance for every VLAN. Now MST the industry standard, with that you can group multiple VLANs into the same spanning three instance. But with the Cisco versions, PVST Plus and Rapid PVST Plus, they use a separate spanning three instance for every single individual VLAN.

So looking at the load balancing with PVST Plus or Rapid PVST Plus, same example that we covered before and again here, CD One is going to be made the root bridge for VLANs ten to 19. CD Two is the root bridge for VLANs 20 to 29. So VLANs ten to 19 go over the left hand path up to CD One. VLANs 20 to 29 go over the right hand path to CD Two. So so far it’s looking exactly the same as MST. The difference is with MST, we grouped the VLANs.

So with MST we had one group going up the left hand side, we had another group going up the right hand side. So we had two spanning three instances. With PVST plus and rapid PVST plus you can’t group the VLANs. You have a separate instance for each one. So rather than having two total instances like we had with MST here, we’re going to have 20 separate instances, one for each individual VLAN. So the Cisco versions, PBST Plus and Rapid PBSD Plus, they put a bit more load on the switch because it has to calculate spanning three instances at the VLAN level rather than being able to do it at the group level.

Okay, so those are the different versions of Spanning tree. For which versions will be supported on your switch? It depends on the particular model of switch that you’re using. PVST plus will always be supported. That will be the default. It will usually also support rapid PVST plus as well. And possibly depending on the model of switch, it might also support MST, the open standard, multiple spanning tree. One last thing to tell you. PVST plus, which is the default on Cisco switches, will assign the route designated or alternate role to ports. We spoke about our root and are designated in our blocking ports in the last lecture. Just giving you terminology here. The alternate parts are your blocking parts with PVST Plus. Okay, that was the whole thing. We’ll look next at how to actually monitor and verify Spanish fee in the next lecture.

7. Verification – show spanning-tree

You’ll learn how to verify the spanning tree protocol. I’m using the same network topology that I’ve been using throughout the rest of the section. So we’ve got the layer three part of the network up at the top with our routers r one and R two and going northbound. And then we’ve got the layer two part of the network with our core distribution switches CD one and CD two and our access layer switches access three and access four. There’s obviously layer three connections going from the PCs up to the routers as their default gateways as well. So what we want to do here is to map out how the spanning tree has been configured. In this example the switches have been configured with VLANs but spanning tree has not been configured at all. So they’re all going to be using the default priority. So what we want to do here is to determine which is the root bridge first. Then from there we can figure out our root ports on the other switches, our designated ports and our blocking ports so that we can check if it spanning tree has eliminated any loops in the layer two part of the network.

And we can also see the tree that traffic is going to be traveling over. Now the diagram here is a screenshot from packet tracer and I’ve got the link lights enabled so you can actually see easily where the root bridge is and the path that traffic is going to go over just from looking at it here you see that both access three and CD two their links are all green. So one of those two is going to be the root bridge and on CD one it’s blocking a port going towards CD two. So CD two can’t be the root bridge. It’s going to be access free. I can see on access free all the links going to it are green on both sides. I can also see from the diagram that the ports that are being blocked are gag two on CD one and portfast 00:21 on axis four. So both of the possible loops going from CD one to CD two, axis three that has been broken by blocking GIGO two on CD one and the potential loop between CD one, CD two and axis four has been broken by blocking the portfast 00:21 on access four.

Okay, so I can see all that from the diagram but obviously in the real world you’re not going to have a diagram which shows you exactly how spanning tree is configured. So how do we figure out how the spanning tree is laid out in a production network? That’s what we’re going to cover here using the same example topology. So really the Swiss army knife command for checking your spanning tree configuration is show spanning tree. You already know that the default spanning C version on a Cisco switch is PVST Plus which runs a separate spanning tree instance for every VLAN. So you also need to specify the VLAN as well. In the example here, we’re running the command first off on the root bridge which was on access three for our example. So I say show spanning three VLAN one. So you have to well if you don’t specify the VLAN, it will show you the spanning three for all of your different VLANs. And if you’ve got a lot of VLANs on the switch, it’s going to be very long output. So you want to specify the particular VLAN. The next thing you can see here is that the protocol is Ieele and it’s not actually using one of the standard IEEE spanning three versions, it’s using Cisco’s proprietary PVST.

Plus it’s just a quirk of the history of how this was developed with Cisco called PBS T-I-E when you use the show spanning tree command. So using the default PBST plus here next thing to tell you about the output of the command, there’s two sections, the root ID section and the bridge ID section. The root ID gives you information about the root bridge. The bridge ID section gives you information about this switch. So the root ID information should be similar on all of the switches in your local area network. The bridge ID section will specify the Mac address for that individual switch.

Next thing we’re on the route bridge here, we can see that very clearly under the root ID section. It tells us this bridge is the root and that’s why the Mac address is the same in the root ID section and in the bridge ID section, because this switch is the root bridge. And notice that for this example, the switches Mac address ends in D 43 D. That’s important. When we look at the information coming up on the next switch we’ll look at which is a non root bridge.

So D four 3D, we can see the priority in here, the priority is 32768, which is the default priority. This has been elected as the root bridge. So I can see very simply from misinformation that all my bridges, all my switches must be set with the default priority which is 32768, and that this switch was elected as the root bridge because it’s got the lowest Mac address.

The last thing to see on the output of the command is it gives you the status of all your interfaces that are connected to other switches. Because this is the root bridge, all our ports are going to be designated ports and forwarding. Okay, next let’s look at the output on a non route bridge. So that was on axis three. Next up we’ll look at the output on CD one, and from the diagram we can see it is forwarding on interfaces fast 00:24 and 00:21 and it’s blocking on interface gig zero two. So looking at the output on CD one, I do a show spanning three for VLAN one. Again, I can see that this switch is also running PVST.

Plus you want all the switches in your network to be running the same spanning tree version. Again, we’ve got the root ID and the bridge ID section. And because this is not the root bridge, the two Mac addresses are different. Now again, the route ID section gives you information about the root bridge. You want all of the switches in your network for the same VLAN to be agreeing on which switch this is. And we can see that it is the same D four 3D.

So that looks good. In the bridge ID section I can see that this switch’s unique Mac address ends in 3902. This switches mark address starts with zero zero 90, which is higher than the root bridges mark address of 0001. That’s why the root bridge was preferred over this one. Other information in the root ID section I can see that this switch’s cost to get to the root bridge is 19 and the root port is interface fast ethernet 00:24.

That’s the only cost path interface to get to the root bridge. And down at the bottom I can see the interface gig zero two. Its role is alternate, so it is a blocking part. It’s a part that has been selected to block a potential loop. Interfaces Fast 00:21 and 00:24 are designated in a root port and they are both forwarding. So that was CD one. If we look at the topology diagram again, let’s also have a look at CD two. And on CD two all its interfaces should be forwarding.

So let’s jump into the lab to see this. I will go to my enable prompt and show spanning tree for VLAN one. And in here I can see that it agrees that the root bridge is access free, ending with Mac address D four 3D. This switches Mac address also begins with zero zero 90, so it’s a higher Mac. That’s why it was not selected as the root bridge. All of my switches are running the default priority of 32768 for this switch to get out to the root bridge. It uses interface fast ethernet 21 and the cost is 19. I can see all of my ports that are connected to other switches down at the bottom here and Fast 00:21 again is the root part. The other two ports are designated ports, so all of these ports are forwarding.

Finally, let’s look at the topology diagram again. The last switch to look at is access four, which is forwarding on Fast 00:24. That is the root part and it’s blocking on Fast 00:21. So let’s jump on to access four. In the lab show spanning three four VLAN one I can see. It also agrees that the root bridge is access free. This switches Mac address begins with zero zero 60, which is higher than the root bridges Mac address. All of my switches are using a product a 32768. The root port is fast 00:24. And the cost to get to the root bridge is 38. We’re forwarding on portfast 00:24, and we’re blocking on portfast 00:21. So that’s how you can check your spanning tree topology. There’s not really a quick way of doing this. If you just have command line access to your switches, jump onto one of your switches and do show spanning tree there that will tell you which is the root bridge. To find the entire topology and to see which blocks or which parts are forwarding and which are blocking, you really just have to map it out switch by switch. So it’s handy if you use a pencil and paper for this. You can draw it down in diagram everything. Okay, so that was the show spanning tree command.