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Alright, now we're moving on to the second challenge task, which is saying that

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the root

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bridge has been lost.

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And so I've already modified the diagram here.

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It basically just creates a brand new topology for us to explore.

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It's just going to be a little bit more simple without all of those extra links

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and without

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the root bridge in the middle.

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In fact, a lot of ways just kind of goes back to the same type of topology we

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've spent

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a lot of time with, which is that square topology.

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The difference is we've got a lot of these different types of costs in here.

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We still have the double link to figure out.
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We still have the double link to figure out.

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And so once again, if we're going to start going down the path of spanning tree

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topologies,

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it always has to start with where the root bridge is.

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Now before it was easy, we had a root bridge right in the middle with a 4k

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priority that's

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as low as it gets outside of a zero priority, which by the way is perfectly

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legal.

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We might see zeros out on the network, but either way, we're not that low

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anymore.

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We're talking about 16 being the lowest and there's two of them.

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So now we do need to think about how bridge priorities really interact with one
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So now we do need to think about how bridge priorities really interact with one

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another

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and ultimately how they tie into the bridge ID.

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So always remember the bridge ID is different than the bridge priority.

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The bridge ID is the, remember that big word concatenation, sort of an addition

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of the

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priority with the MAC address.

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And so it basically just forms one very long number, but the priority comes

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first.

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It's the higher ordered bits.

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And so at that point, we can look at the MAC address because the priority is
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And so at that point, we can look at the MAC address because the priority is

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the same

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in this situation.

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And we want to always with spanning tree, remember it's the lowest.

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So we had to look at the lowest MAC address.

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And if a MAC address starts with different hex characters, then we only need to

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evaluate

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those.

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We go to zero E compared to zero three, E is once again, hex for 14, three is

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way lower

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than 14.

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And therefore it's going to win.

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So switch four down here becomes our root.
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So switch four down here becomes our root.

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Now this is going to completely change the topology because before switch four

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was blocking

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one of these links and we don't need to worry about that anymore because switch

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four is

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the root pretty fascinating.

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So let's start with switch four being the root having all of the designated

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ports.

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And from there we can work our way out.

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So we know that switch two is going to have to block one of these links.

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Now it's based on the lower and higher port numbers on switch four because it's
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Now it's based on the lower and higher port numbers on switch four because it's

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upstream.

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These are both going to be upstream ports.

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If we have multiple upstream ports, one of them has to be the root port and one

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of them

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has to be the, well, all the rest have to be non-designated.

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And so it's going to be based on the fact that this is the higher port and

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therefore

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it gets, let me just encircle that.

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So port zero here is the one that's going to be placed in to the blocking state

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.

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So that becomes our non-designated port.
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So that becomes our non-designated port.

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Meanwhile we have a designated port here.

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Now technically we did skip the step of saying, hey, we've got a cost of four

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to the root

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this way.

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What about this way?

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Well, we can tell that that's going to be a much longer path and just the fact

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that it's

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32 to start with.

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So we don't need to worry about a situation where we'd be blocking both of

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these and making

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that our route instead.

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In fact, I said that was a designated port.

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This should be a root port.

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So that would be our root port.

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Again, it's an upstream port facing the route.
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Again, it's an upstream port facing the route.

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So this is going to go a whole lot faster than the last one because at this

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point we

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just have to figure out a couple of these other links.

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Now with switch five we can start here because we just need to know, hey, is

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this going to

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be my root port or is this going to be my root port?

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10 to the root this way, 36 to the root that way makes it pretty clear that the

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best cost

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is directly connected to switch four.

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So that's my root port.

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And now we need to just figure out who's blocking on these other links.
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And now we need to just figure out who's blocking on these other links.

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So let's go ahead and take a look at switch five since we're here.

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We're going to take a look at this link.

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Now again, we're looking at the path cost to the root.

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So switch five has a path cost of 10 and we would advertise a path cost of 10.

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Now if switch three was saying, hey, I've got another path to the root, what

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would that

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cost be?

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It would be 36.

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Now maybe switch three decides to go this way to the root.
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Now maybe switch three decides to go this way to the root.

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But if it goes that way to the root, this is an upstream port, not a downstream

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.

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And so we don't need to worry about that scenario.

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Plus the fact it would be going through switch five immediately means it's a

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worse path than

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what switch five has because we'd have to add the cost of four.

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So at this point, we can very clearly tell switch five is going to win this

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because it

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has a cost of 10 where a switch three, if it's going the other way, has a much

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larger

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cost.

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So switch five is going to be designated.

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Now we could say switch three is a non designated port here, but we have to
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Now we could say switch three is a non designated port here, but we have to

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wait for a second.

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We don't know whether switch three is going to block this link or possibly

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block this

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link.

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It can't block them both.

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And so if it's possible, this becomes a root port, we wouldn't want to block it

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.

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So all we can say for sure is that this is an upstream port.

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Let's go ahead and explore the other path and figure out whether or not it's

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upstream

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or downstream to switch to.

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So we're going to have a very similar exchange here.

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Switch two is going to advertise a cost of four.
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Switch two is going to advertise a cost of four.

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And by the way, because switch two has a root port, we know that it has a path

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cost of four

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and therefore it sends four now switch three.

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If it was to say, hey, I've got a if this is my root, my root port and it's

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advertising

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it, it's going to advertise 14.

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And so it would say, hey, I've got a path cost of 14.

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So it's going to lose this battle, so to speak, meaning that switch two is

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going to set this

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as designated and this is another upstream port on switch three.
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as designated and this is another upstream port on switch three.

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So this is where switch three is going to have to make a decision.

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We have a path this way to the root and we have a path this way to the root.

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How do we make that decision?

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It starts with cost.

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It always starts with cost.

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I want to emphasize that we could easily jump in and say, well, switch two has

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a bridge

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ID or bridge priority of 16 K and switch five has a bridge priority of 28 K. We

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don't go

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to the tie breakers unless cost is tied.

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And so we have, I mean, I guess maybe that's straightforward.
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And so we have, I mean, I guess maybe that's straightforward.

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We don't use tie breakers unless there's a tie.

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Cost is always going to be considered first.

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It will always take the best path from a cost perspective.

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So let's focus in on that first.

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And we see that we've got a cost this way of 36, not looking good.

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And then this path cost we already said is 14.

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So 14 versus 36 now we can say I would, I'm going to prefer this path as my
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So 14 versus 36 now we can say I would, I'm going to prefer this path as my

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root port,

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which means because we've already lost the battle this way, this becomes my

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other, my

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other upstream port.

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It actually becomes non designated, which means it's blocking.

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So we only have two blocking ports in this topology.

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We have the link here going from switch three up to switch two and then switch

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two has to

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block one of these connections because it is double connected to switch four.

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So this is what would happen if switch one went down.

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Now if switch one comes back online, I'd say miraculously, but switches reboot
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Now if switch one comes back online, I'd say miraculously, but switches reboot

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all the

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time.

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It's not much of a miracle.

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It comes back online.

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It will establish the entire original topology.

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And the reason for that is because spanning tree protocol is pretty basic when

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it comes

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to root election switch four or switch one is going to come back online with a

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priority

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of four thousand for 40 96.

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And it will take over as roots, which four will no longer be the roots, which

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one will

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become the root and everything's going to go into mass chaos.
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become the root and everything's going to go into mass chaos.

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Every port is going to go back into listening until we get all of these port

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roles and port

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states reestablished.

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But for the scenario where switch one remains offline, we have our chosen top

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ology.

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Now we do have one final challenge task, I suppose, task number three, which is

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what

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configuration can be placed here on the switch four in order to change the

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blocking decision

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being made by switch two.
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being made by switch two.

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So we see switch two is only making one blocking decision.

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It is blocking the link to switch four that is the higher link on switch four.

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But again, just like with our bridge ID, we do have the concept of the port ID

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and that

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consists of a priority and then the port number.

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So if the priority is the same, which again, by default, it's 128 and we didn't

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give port

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priority, so I have to assume they're all the defaults.
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priority, so I have to assume they're all the defaults.

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Well, if they're all tied, that's where we have to compare the number.

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But if there is a priority, we don't have to compare the numbers anymore.

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So we typically have two choices in this case, I want this to be the preferred

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path.

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Therefore, I can either lower the priority on this side, remember lower wins,

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so I can

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make it, let's say 80 or something like that.

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Or on this lower port on the right, I can increase the priority number.

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And so I can make it like 160 or 144 to 16 more.
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And so I can make it like 160 or 144 to 16 more.

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Just remember that we have to configure these in increments of 16 on Cisco

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devices.

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So either one of these situations will make it so that these two port roles are

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flipped

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because now this will be the favored side, meaning that that will be switched

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to root

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port and the other side will be placed into non-designated port role.

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So this is just one large example admittedly of calculating outstanding tree
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So this is just one large example admittedly of calculating outstanding tree

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topologies.

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It starts with identifying the root and then we just go from there, I've said

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this at the

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end of the last video, we just look for as many designated ports and root ports

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as we

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can.

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And then we start comparing path costs, always remember path costs win.

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That's always the first criteria, we want to take the fastest path to the root.

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And from there we can start doing those tie breakers like comparing bridge

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priorities

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and, well, bridge IDs and port IDs and such.
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and, well, bridge IDs and port IDs and such.

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But take a moment to draw out your own topology, add some priorities, add some

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MAC addresses,

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add some link costs.

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It doesn't take much just to throw a topology together on paper and then play

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around with

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it and see if you can figure out that topology.

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It goes a long way towards making us just establish that critical thinking of

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do I know

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all of the steps that's required in order to calculate all of this out.

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And then even better, if you do have a lab environment where you can emulate

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that topology,
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that topology,

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throw it in there and make sure that everything that you had calculated out is

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actually correct.

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I hope this has been informative for you and I'd like to thank you for viewing.