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(electronic sound)

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- [Instructor] So here we are in Part 4,

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which will be the last section.

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of this CCNA Switching
Lab that we've built.

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And now we're gonna modify
some Spanning-Tree parameters.

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Because, as we talked about
at the end of the last video,

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currently with the way Spanning-Tree is,

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is that everything being sent from

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Router 1 across these switches,

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is going across our lowest
numbered interfaces,

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1/9, but the objective said that

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it should be taking this path.

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So, what are we gonna do to fix that?

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Alright, well step
number one is, we have to

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identify who is the root
bridge in this particular case.

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Well just by looking at this, hopefully

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you can answer that question.

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Only the root bridge would
have all of his ports

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in the forwarding state,
well that's actually

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not technically true, but in this

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particular case that's the
case, how do we verify that?

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Well, let's go onto Ether Switch Router 1.

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And we can do the "sho
spanning-tree" command.

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Now if this was a real
switch you'd have to do

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"sho spanning-tree" and
then type a vlan number,

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so let's go ahead and do that.

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Sho spanning-tree for vlan 4.

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And right here it says, "we are

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"the root of the spanning tree".

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And if we do that for vlan
5, it'll be exactly the same:

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"we are the root of the spanning tree".

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Now when it comes to vlan load balancing,

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the first thing you should look at is,

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if I move my root bridge around,

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could I possibly get what I wanna get?

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Could I meet my objective simply by

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moving the root bridge around?

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In this particular case,
that's not gonna help us.

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Because if I made the switch on the bottom

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the root bridge, let's just
say theoretically I did that.

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And let's just say I did
it for, now I think it said

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the lab objectives here
did not really specify

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which vlan we're talking about, so let's

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say I made him the root
bridge for everything.

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Well all that would do
is what we would see is

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now all of his ports would go forwarding,

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because the root bridge,
every port is forwarding

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as a designated port, and
then the switch on the top,

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he would end up saying,
"well I'm gonna go ahead

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"and go forwarding on
my lowest numbered port

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"and I'm gonna go blocking on this port".

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Not gonna go into the
details of why, if you're

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not sure of how that
happens, please look at my

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CCNE video series on routing and switching

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where I go into gory
details on spanning tree.

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So moving the root bridge
around, eh, not gonna help us.

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So might as well get rid of that.

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Let's go back to where we were.

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Alright, so ultimately,
here's what we need to do.

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We need to somehow trick
Ether Switch Router 2.

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We need to make him believe that this

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interface here should be his root port.

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We want him to select
this as the root port,

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so that he'll go forwarding, so

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how do we make him believe that?

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Well, we actually have two
ways that we can do it.

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Number one, remember that
spanning tree ultimately

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relies on cost to figure out the best

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path through a network,
so one thing we could do

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is we could go onto his
interface and tell him,

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"Hey, the spanning tree
cost of this interface

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"is less than the spanning
tree cost of 1/9."

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That's one thing we could do.

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Alternatively, we could
leave the cost alone

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because right now they're
exactly identical,

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because when the cost is identical,

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what ultimately ended
up causing this scenario

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was the port priority, the port priority.

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You see, in this particular case,

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when BPDUs came downstream this way,

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both those BPDUs had the same bridge ID,

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because they both came
from the same bridge.

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They both had the same
cost, which was zero,

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because they were coming
from the root bridge.

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The only thing that was
different about them

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was that the port IDs were different.

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Now remember, port ID and spanning tree is

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a two-part field: you have a port priority

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and a port ID, a port identifier.

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Well by default, the port
priority is the same.

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It doesn't matter. It's always the same.

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So if the port priority is the same,

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the only thing that made these unique

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were the port numbers, and 1/9, as a

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lower number than 1/15, was preferred.

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Now you can't change the port number,

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but you can change the port priority.

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So let's do both, let's go
ahead and modify the cost,

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and then we'll set it back to default,

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and then we'll modify the port priority.

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So if I wanna modify the cost,
I'm gonna wanna modify the

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cost on this port right
here, 1/15 on Ether Switch 2.

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Alright, so let's get
out of Ether Switch 1.

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Go over here to Ether Switch 2.

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Let's go into interface
fast ethernet 1/15,

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spanning-tree, vlan 4, and now
we're gonna wanna say cost,

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and, well what's the cost right now?

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Well you should know
for the CCNA, the cost

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of a fast ethernet link is always 19.

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If you're using regular 802.1D,

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that's the default port cost.

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So let's do spanning-tree,
vlan 4, cost, how about 18.

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And let's do that same
thing for vlan 5 as well.

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And now when we do show spanning tree,

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for vlan 4, remember,
who are we on right now?

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We're on the bottom switch right here.

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And what did it look like previously?

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Previously 1/15 was in the blocking state,

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but now what we've done, is we've said,

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okay, your cost to get to the root bridge

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on 1/9 is 19, 'cause it's fast ethernet,

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But your cost to get it to the root bridge

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over here is 18, we lowered it.

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So now ultimately we
should see these two swap.

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We should see 1/9 go
into the blocking state,

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and we should see 1/15 go
into the forwarding state.

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And there we go, 1/9
for vlan 4 is blocking,

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1/15 for vlan 4 is forwarding.

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And we'll see the exact
same output for vlan 5,

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because we modified the
cost for vlan 5 as well.

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1/9's blocking, 1/15 is forwarding.

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So if I scroll down here, you'll see that

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on 1/9 we didn't do anything, we left it

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to its defaults, but on the
interface going upstream,

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to the root bridge, we
modified the cost on 1/15.

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So that was one way that you could

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accomplish this objective,
the other way was

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to leave cost alone, so let's
go ahead and take this out.

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The other way was to go
to the upstream switch

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that's sending you the
BPDUs, and modify his

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port priority, so let's
go to Ether Switch 1,

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interface fast ethernet
zero slash, actually before

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we go there, show spanning-tree vlan 4.

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And we can see here, that they both have

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the exact same port priority of 128.

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That's our default port priority.

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So now what we're gonna do is we're

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gonna lower his port priority to 127,

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which will make the guy
downstream from him,

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which is Ether Switch 2,
prefer fast ethernet 1/15.

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Spanning-tree, and I'll
just do, I'll just do it

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for everything, I'm not gonna specify

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any particular vlan, although I could.

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Port priority 127, ah,
but we have to do it

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in increments of eight, so we'll do 120.

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So now what's happening,
is we're back to this,

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where on the bottom switch we have 19 19,

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but the BPs are coming
down from Ether Switch 1,

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this one has a port priority of 128,

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this one has a port priority of 127,

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and remember in spanning
tree, lower is always better.

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And so now that will cause Ether Switch 2

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to swap out these numbers
and now this will go blocking

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and this will go forwarding,
so let's verify that.

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Let's go back to Ether
Switch 2 and make sure that

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1/15 is now in the forwarding stage.

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And there we can see,
the lower number port

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is now blocking, the higher
number port is now forwarding.

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And that concludes this
CCNA level switching lab.

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(electronic sound)