Alright, now we're moving on to the second challenge task, which is saying that
the root
bridge has been lost.
And so I've already modified the diagram here.
It basically just creates a brand new topology for us to explore.
It's just going to be a little bit more simple without all of those extra links
and without
the root bridge in the middle.
In fact, a lot of ways just kind of goes back to the same type of topology we
've spent
a lot of time with, which is that square topology.
The difference is we've got a lot of these different types of costs in here.
We still have the double link to figure out. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We still have the double link to figure out.
And so once again, if we're going to start going down the path of spanning tree
topologies,
it always has to start with where the root bridge is.
Now before it was easy, we had a root bridge right in the middle with a 4k
priority that's
as low as it gets outside of a zero priority, which by the way is perfectly
legal.
We might see zeros out on the network, but either way, we're not that low
anymore.
We're talking about 16 being the lowest and there's two of them.
So now we do need to think about how bridge priorities really interact with one WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So now we do need to think about how bridge priorities really interact with one
another
and ultimately how they tie into the bridge ID.
So always remember the bridge ID is different than the bridge priority.
The bridge ID is the, remember that big word concatenation, sort of an addition
of the
priority with the MAC address.
And so it basically just forms one very long number, but the priority comes
first.
It's the higher ordered bits.
And so at that point, we can look at the MAC address because the priority is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so at that point, we can look at the MAC address because the priority is
the same
in this situation.
And we want to always with spanning tree, remember it's the lowest.
So we had to look at the lowest MAC address.
And if a MAC address starts with different hex characters, then we only need to
evaluate
those.
We go to zero E compared to zero three, E is once again, hex for 14, three is
way lower
than 14.
And therefore it's going to win.
So switch four down here becomes our root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So switch four down here becomes our root.
Now this is going to completely change the topology because before switch four
was blocking
one of these links and we don't need to worry about that anymore because switch
four is
the root pretty fascinating.
So let's start with switch four being the root having all of the designated
ports.
And from there we can work our way out.
So we know that switch two is going to have to block one of these links.
Now it's based on the lower and higher port numbers on switch four because it's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now it's based on the lower and higher port numbers on switch four because it's
upstream.
These are both going to be upstream ports.
If we have multiple upstream ports, one of them has to be the root port and one
of them
has to be the, well, all the rest have to be non-designated.
And so it's going to be based on the fact that this is the higher port and
therefore
it gets, let me just encircle that.
So port zero here is the one that's going to be placed in to the blocking state
.
So that becomes our non-designated port. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So that becomes our non-designated port.
Meanwhile we have a designated port here.
Now technically we did skip the step of saying, hey, we've got a cost of four
to the root
this way.
What about this way?
Well, we can tell that that's going to be a much longer path and just the fact
that it's
32 to start with.
So we don't need to worry about a situation where we'd be blocking both of
these and making
that our route instead.
In fact, I said that was a designated port.
This should be a root port.
So that would be our root port.
Again, it's an upstream port facing the route. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Again, it's an upstream port facing the route.
So this is going to go a whole lot faster than the last one because at this
point we
just have to figure out a couple of these other links.
Now with switch five we can start here because we just need to know, hey, is
this going to
be my root port or is this going to be my root port?
10 to the root this way, 36 to the root that way makes it pretty clear that the
best cost
is directly connected to switch four.
So that's my root port.
And now we need to just figure out who's blocking on these other links. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And now we need to just figure out who's blocking on these other links.
So let's go ahead and take a look at switch five since we're here.
We're going to take a look at this link.
Now again, we're looking at the path cost to the root.
So switch five has a path cost of 10 and we would advertise a path cost of 10.
Now if switch three was saying, hey, I've got another path to the root, what
would that
cost be?
It would be 36.
Now maybe switch three decides to go this way to the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now maybe switch three decides to go this way to the root.
But if it goes that way to the root, this is an upstream port, not a downstream
.
And so we don't need to worry about that scenario.
Plus the fact it would be going through switch five immediately means it's a
worse path than
what switch five has because we'd have to add the cost of four.
So at this point, we can very clearly tell switch five is going to win this
because it
has a cost of 10 where a switch three, if it's going the other way, has a much
larger
cost.
So switch five is going to be designated.
Now we could say switch three is a non designated port here, but we have to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now we could say switch three is a non designated port here, but we have to
wait for a second.
We don't know whether switch three is going to block this link or possibly
block this
link.
It can't block them both.
And so if it's possible, this becomes a root port, we wouldn't want to block it
.
So all we can say for sure is that this is an upstream port.
Let's go ahead and explore the other path and figure out whether or not it's
upstream
or downstream to switch to.
So we're going to have a very similar exchange here.
Switch two is going to advertise a cost of four. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Switch two is going to advertise a cost of four.
And by the way, because switch two has a root port, we know that it has a path
cost of four
and therefore it sends four now switch three.
If it was to say, hey, I've got a if this is my root, my root port and it's
advertising
it, it's going to advertise 14.
And so it would say, hey, I've got a path cost of 14.
So it's going to lose this battle, so to speak, meaning that switch two is
going to set this
as designated and this is another upstream port on switch three. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
as designated and this is another upstream port on switch three.
So this is where switch three is going to have to make a decision.
We have a path this way to the root and we have a path this way to the root.
How do we make that decision?
It starts with cost.
It always starts with cost.
I want to emphasize that we could easily jump in and say, well, switch two has
a bridge
ID or bridge priority of 16 K and switch five has a bridge priority of 28 K. We
don't go
to the tie breakers unless cost is tied.
And so we have, I mean, I guess maybe that's straightforward. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so we have, I mean, I guess maybe that's straightforward.
We don't use tie breakers unless there's a tie.
Cost is always going to be considered first.
It will always take the best path from a cost perspective.
So let's focus in on that first.
And we see that we've got a cost this way of 36, not looking good.
And then this path cost we already said is 14.
So 14 versus 36 now we can say I would, I'm going to prefer this path as my WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So 14 versus 36 now we can say I would, I'm going to prefer this path as my
root port,
which means because we've already lost the battle this way, this becomes my
other, my
other upstream port.
It actually becomes non designated, which means it's blocking.
So we only have two blocking ports in this topology.
We have the link here going from switch three up to switch two and then switch
two has to
block one of these connections because it is double connected to switch four.
So this is what would happen if switch one went down.
Now if switch one comes back online, I'd say miraculously, but switches reboot WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now if switch one comes back online, I'd say miraculously, but switches reboot
all the
time.
It's not much of a miracle.
It comes back online.
It will establish the entire original topology.
And the reason for that is because spanning tree protocol is pretty basic when
it comes
to root election switch four or switch one is going to come back online with a
priority
of four thousand for 40 96.
And it will take over as roots, which four will no longer be the roots, which
one will
become the root and everything's going to go into mass chaos. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
become the root and everything's going to go into mass chaos.
Every port is going to go back into listening until we get all of these port
roles and port
states reestablished.
But for the scenario where switch one remains offline, we have our chosen top
ology.
Now we do have one final challenge task, I suppose, task number three, which is
what
configuration can be placed here on the switch four in order to change the
blocking decision
being made by switch two. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
being made by switch two.
So we see switch two is only making one blocking decision.
It is blocking the link to switch four that is the higher link on switch four.
But again, just like with our bridge ID, we do have the concept of the port ID
and that
consists of a priority and then the port number.
So if the priority is the same, which again, by default, it's 128 and we didn't
give port
priority, so I have to assume they're all the defaults. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
priority, so I have to assume they're all the defaults.
Well, if they're all tied, that's where we have to compare the number.
But if there is a priority, we don't have to compare the numbers anymore.
So we typically have two choices in this case, I want this to be the preferred
path.
Therefore, I can either lower the priority on this side, remember lower wins,
so I can
make it, let's say 80 or something like that.
Or on this lower port on the right, I can increase the priority number.
And so I can make it like 160 or 144 to 16 more. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so I can make it like 160 or 144 to 16 more.
Just remember that we have to configure these in increments of 16 on Cisco
devices.
So either one of these situations will make it so that these two port roles are
flipped
because now this will be the favored side, meaning that that will be switched
to root
port and the other side will be placed into non-designated port role.
So this is just one large example admittedly of calculating outstanding tree WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is just one large example admittedly of calculating outstanding tree
topologies.
It starts with identifying the root and then we just go from there, I've said
this at the
end of the last video, we just look for as many designated ports and root ports
as we
can.
And then we start comparing path costs, always remember path costs win.
That's always the first criteria, we want to take the fastest path to the root.
And from there we can start doing those tie breakers like comparing bridge
priorities
and, well, bridge IDs and port IDs and such. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and, well, bridge IDs and port IDs and such.
But take a moment to draw out your own topology, add some priorities, add some
MAC addresses,
add some link costs.
It doesn't take much just to throw a topology together on paper and then play
around with
it and see if you can figure out that topology.
It goes a long way towards making us just establish that critical thinking of
do I know
all of the steps that's required in order to calculate all of this out.
And then even better, if you do have a lab environment where you can emulate
that topology, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that topology,
throw it in there and make sure that everything that you had calculated out is
actually correct.
I hope this has been informative for you and I'd like to thank you for viewing.