All right, let's go and jump into this lab task.
We want to make sure we know how to manipulate this
spanning tree topology, whether it's inbound or outbound methods,
and then we want to check port fast as well.
So challenge task number one here.
Ensure that switch five chooses the path through switch four.
That'd be gig three slash one here.
And we want to make sure that is the root port.
Now, given that our BPDs are flowing downstream from the root,
which we might want to do a discovery to make sure we know which
switch is the root.
It'll be switch one in this case. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It'll be switch one in this case.
From there, we can understand that we probably
need to use an inbound method here.
I mean, we're really only using port priorities
in the situation up here where we have multiple ports
between two switches.
But now we've got switches due to different switches upstream.
And so looking at this, we want to do a discovery.
We want to make sure that we're not making bad assumptions.
But also, my starting assumption would
be that this port is probably in a blocking state.
Simply because it's further away from the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Simply because it's further away from the root.
Now, if we look at things just from a hop perspective,
we go to switch three, and then we
see switch five and switch four are the same distance
from the root, at which point switch four could also
be blocking.
And so let's go ahead and go out and take a look
and see what exactly is happening in the lab.
I'm going to go to switch five, do a show, spanning tree,
blocked ports.
And we do see that gig three slash one,
that link on the left is in the blocking state.
So I can do a number of things here.
First of all, I could do a show spanning tree, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
First of all, I could do a show spanning tree,
and I could take a look at the information, especially
what my cost to the root is.
It says it's eight.
And I could also do a show spanning tree, pick a VLAN,
and then say root.
And I get the same information.
My root cost is eight.
So a couple of different places to discover that.
So looking back at the diagram here,
given that gig interfaces have a path cost of four,
it's probably pretty safe to look at this and say, OK,
so it's probably four and four is my eight. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
so it's probably four and four is my eight.
And I can verify that.
I don't have to assume anything.
When I look up here, for example, gig two slash one
and three slash one, it shows that it has path cost of four
on that link.
And then I could go to switch to and verify,
although given that there's only one link left in the path,
it's got to be four, given that the total path cost is eight.
Now that configuration also told us that this path cost is four.
And we could probably go out and verify this,
but I will just say that this path link here is also four.
We left all the costs default. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We left all the costs default.
So if I want to prefer this path versus this winding path,
well, then I'm going to have to compare eight versus 12,
which means I'm going to have to either seriously decrease
this, which is not mathematically possible.
I can't make it so that it's negative,
because I'm already looking at a path cost of eight this way.
I'd have to make this somehow negative to get that below eight.
So what I'm going to have to do is increase the cost on this link.
And I need to make sure that the total path cost this way WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And I need to make sure that the total path cost this way
is greater than this direction.
So basically, I need to make it so that the total path cost
is greater than eight.
So I'm going to go ahead and get on here to interface gig
two slash one, that'd be the link on the left.
I'm going to say spanning tree cost,
and I'll just add 10 to it.
That should make the total path cost 14.
So while spanning tree sorts that out,
we basically turn this into 10.
So now this should be 14, and this will still be 12. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So now this should be 14, and this will still be 12.
So let's go and confirm that the blocked port moved.
We're expecting this is no longer blocking,
but this should be blocking gig two slash one,
and show spanning tree blocked ports.
Indeed shows gig two slash one.
So on to task number two, and task number two
is actually a little bit diabolical.
So if you got it, then really good job,
because we're going to have to use all of our knowledge
to solve task two.
Task two says that switch four wants us to pick gig zero two WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Task two says that switch four wants us to pick gig zero two
as the root port, and we're not to configure switch four
at all for this.
So we might immediately say, OK, well, good.
If I can't configure switch four,
I know I can push policy down via the port priority here
from switch two.
Now we have to look at this and say,
which one are we expecting to have the port blocked?
I might say that, well, gig zero two is already lower
than gig zero three, and so that should be the blocking
port on the left.
But remember, it's from the perspective
of the upstream device when we're talking
about port priorities. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
about port priorities.
It's an outbound concept.
So it's actually gig zero two that's preferred,
which means that probably gig, well, gig zero three
from the switch two's perspective,
but gig zero two on the other side of the link
is probably in the blocking state.
And now this is an assumption.
So let's go ahead and go and take a look at switch four
and find out if our assumption is valid.
So show spanning tree blocked ports.
Now look at that.
Our assumption is not valid.
This is where this lab task gets a little bit diabolical. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is where this lab task gets a little bit diabolical.
So we actually find that both of these ports
are being blocked, which means that gig one zero must actually
be the root.
Now the root port here is determined by cost.
And so therefore, we must assume that either the costs are--
is better this way, or it's a tie,
and switch three wins the tiebreaker between switch two.
And that's actually the case in this lab environment.
So this is interesting. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is interesting.
I am not allowed to change switch four's configuration.
And I yet have to make it so that gig zero two here
is the root port.
How do I go about doing that?
Well, the lab task didn't say that I can't configure switch three.
So let's go ahead and head over to switch three
and do a configuration.
Now what can I do in order to make switch four
want to block this port and open up one of these?
Well, think back to what I just said.
In this lab scenario, we have equal costs everywhere, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
In this lab scenario, we have equal costs everywhere,
which means that it's a tie right now.
Eight this way, eight that way, and switch three
just wins the tiebreaker.
So I just have to make sure there's no tiebreaker here.
I need to make sure that the path this way is worse
than the path this way.
But I can't configure switch four for this.
So what am I going to do?
I'm going to configure switch three.
And I want to make sure that it sees that link as a link cost
higher than four. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
higher than four.
And it really could just be anything higher than four.
It could truly be five.
So gig zero two on switch three interface gig zero two,
I'm going to do spanning tree VLAN one cost.
And let's just do five.
Let's just see if that actually works.
So let's head over to switch four and find out if that worked.
So I'm going to do a show spanning tree blocked ports.
And we are indeed blocking towards switch three now.
And now we just have to resolve the fact
that we don't want to block gig zero two. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we don't want to block gig zero two.
We want to block gig zero three according to our diagram.
Let's just confirm that.
Yes.
So we want to make gig zero to the root port.
The good news is that gig zero three is no longer blocking.
Instead, it's gig one zero that's blocking.
So we're halfway there.
Like I said, this task is a little bit diabolical.
So now we need to worry about the port priority on switch two
to try to make it so gig zero two goes into a forwarding state WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to try to make it so gig zero two goes into a forwarding state
and is no longer blocking.
How do we do that?
Well, again, we favor gig zero two because it's lower.
So maybe we should lower the port priority on gig zero three.
Well, what was that default value?
Again, it was 128.
And so anything less than 128, including 112, for example,
would work just fine here.
It would make gig zero three the favored port over gig zero two.
And then that pushes down and opens up this port.
So let's get on to switch two and configure it
to the port priority of 112 on gig zero three. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to the port priority of 112 on gig zero three.
Switch two, gig zero three.
Switch two, gig zero three, spanning tree VLAN one,
port priority, and then we said 112.
So now let's go to switch four and run that show spanning tree
blocked command and look at that already.
It has moved the block over to gig zero three.
We're still blocking on gig zero one zero that heads to switch
three.
And then as far as this task is concerned, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And then as far as this task is concerned,
we said that we wanted to select gig zero two
as its path to root.
We can always confirm that by using the show spanning tree
root command.
And so my root port is indeed gig zero two.
So that lab task is complete.
Now, lastly, the final lab task is switch five,
making sure no TCNs are sent if gig three slash two
transitions to the forwarding state.
That is simply configuring spanning tree port fast.
So I can say spanning tree port fast on it gig three slash two. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So I can say spanning tree port fast on it gig three slash two.
And that is the end of that task.
So far let's dive algal then task number one.
Just keep in mind that we could have configured
spanning tree port fast.
Well, you know, made that an access port and then pushed it out
by default.
If it was a trunk, we would have needed to specify the trunk
keyword, but we just kept it simple and made sure that we
understand that port fast does indeed squelch those TCNs.
So this lab task, again, or this set of lab tasks again
was a little bit of a challenge.
So hopefully walked through that and maybe flagged this one WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So hopefully walked through that and maybe flagged this one
for later so that you can come back and do this again
and make sure that we've got it all down between cost
and port priority configurations.
I hope this has been informative for you.
And I'd like to thank you for viewing.