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- [Instructor] So here we are in Part 4,
which will be the last section.
of this CCNA Switching Lab that we've built.
And now we're gonna modify some Spanning-Tree parameters.
Because, as we talked about at the end of the last video,
currently with the way Spanning-Tree is,
is that everything being sent from
Router 1 across these switches,
is going across our lowest numbered interfaces,
1/9, but the objective said that
it should be taking this path.
So, what are we gonna do to fix that?
Alright, well step number one is, we have to
identify who is the root bridge in this particular case.
Well just by looking at this, hopefully
you can answer that question.
Only the root bridge would have all of his ports
in the forwarding state, well that's actually
not technically true, but in this
particular case that's the case, how do we verify that?
Well, let's go onto Ether Switch Router 1.
And we can do the "sho spanning-tree" command.
Now if this was a real switch you'd have to do
"sho spanning-tree" and then type a vlan number,
so let's go ahead and do that.
Sho spanning-tree for vlan 4.
And right here it says, "we are
"the root of the spanning tree".
And if we do that for vlan 5, it'll be exactly the same:
"we are the root of the spanning tree".
Now when it comes to vlan load balancing,
the first thing you should look at is,
if I move my root bridge around,
could I possibly get what I wanna get?
Could I meet my objective simply by
moving the root bridge around?
In this particular case, that's not gonna help us.
Because if I made the switch on the bottom
the root bridge, let's just say theoretically I did that.
And let's just say I did it for, now I think it said
the lab objectives here did not really specify
which vlan we're talking about, so let's
say I made him the root bridge for everything.
Well all that would do is what we would see is
now all of his ports would go forwarding,
because the root bridge, every port is forwarding
as a designated port, and then the switch on the top,
he would end up saying, "well I'm gonna go ahead
"and go forwarding on my lowest numbered port
"and I'm gonna go blocking on this port".
Not gonna go into the details of why, if you're
not sure of how that happens, please look at my
CCNE video series on routing and switching
where I go into gory details on spanning tree.
So moving the root bridge around, eh, not gonna help us.
So might as well get rid of that.
Let's go back to where we were.
Alright, so ultimately, here's what we need to do.
We need to somehow trick Ether Switch Router 2.
We need to make him believe that this
interface here should be his root port.
We want him to select this as the root port,
so that he'll go forwarding, so
how do we make him believe that?
Well, we actually have two ways that we can do it.
Number one, remember that spanning tree ultimately
relies on cost to figure out the best
path through a network, so one thing we could do
is we could go onto his interface and tell him,
"Hey, the spanning tree cost of this interface
"is less than the spanning tree cost of 1/9."
That's one thing we could do.
Alternatively, we could leave the cost alone
because right now they're exactly identical,
because when the cost is identical,
what ultimately ended up causing this scenario
was the port priority, the port priority.
You see, in this particular case,
when BPDUs came downstream this way,
both those BPDUs had the same bridge ID,
because they both came from the same bridge.
They both had the same cost, which was zero,
because they were coming from the root bridge.
The only thing that was different about them
was that the port IDs were different.
Now remember, port ID and spanning tree is
a two-part field: you have a port priority
and a port ID, a port identifier.
Well by default, the port priority is the same.
It doesn't matter. It's always the same.
So if the port priority is the same,
the only thing that made these unique
were the port numbers, and 1/9, as a
lower number than 1/15, was preferred.
Now you can't change the port number,
but you can change the port priority.
So let's do both, let's go ahead and modify the cost,
and then we'll set it back to default,
and then we'll modify the port priority.
So if I wanna modify the cost, I'm gonna wanna modify the
cost on this port right here, 1/15 on Ether Switch 2.
Alright, so let's get out of Ether Switch 1.
Go over here to Ether Switch 2.
Let's go into interface fast ethernet 1/15,
spanning-tree, vlan 4, and now we're gonna wanna say cost,
and, well what's the cost right now?
Well you should know for the CCNA, the cost
of a fast ethernet link is always 19.
If you're using regular 802.1D,
that's the default port cost.
So let's do spanning-tree, vlan 4, cost, how about 18.
And let's do that same thing for vlan 5 as well.
And now when we do show spanning tree,
for vlan 4, remember, who are we on right now?
We're on the bottom switch right here.
And what did it look like previously?
Previously 1/15 was in the blocking state,
but now what we've done, is we've said,
okay, your cost to get to the root bridge
on 1/9 is 19, 'cause it's fast ethernet,
But your cost to get it to the root bridge
over here is 18, we lowered it.
So now ultimately we should see these two swap.
We should see 1/9 go into the blocking state,
and we should see 1/15 go into the forwarding state.
And there we go, 1/9 for vlan 4 is blocking,
1/15 for vlan 4 is forwarding.
And we'll see the exact same output for vlan 5,
because we modified the cost for vlan 5 as well.
1/9's blocking, 1/15 is forwarding.
So if I scroll down here, you'll see that
on 1/9 we didn't do anything, we left it
to its defaults, but on the interface going upstream,
to the root bridge, we modified the cost on 1/15.
So that was one way that you could
accomplish this objective, the other way was
to leave cost alone, so let's go ahead and take this out.
The other way was to go to the upstream switch
that's sending you the BPDUs, and modify his
port priority, so let's go to Ether Switch 1,
interface fast ethernet zero slash, actually before
we go there, show spanning-tree vlan 4.
And we can see here, that they both have
the exact same port priority of 128.
That's our default port priority.
So now what we're gonna do is we're
gonna lower his port priority to 127,
which will make the guy downstream from him,
which is Ether Switch 2, prefer fast ethernet 1/15.
Spanning-tree, and I'll just do, I'll just do it
for everything, I'm not gonna specify
any particular vlan, although I could.
Port priority 127, ah, but we have to do it
in increments of eight, so we'll do 120.
So now what's happening, is we're back to this,
where on the bottom switch we have 19 19,
but the BPs are coming down from Ether Switch 1,
this one has a port priority of 128,
this one has a port priority of 127,
and remember in spanning tree, lower is always better.
And so now that will cause Ether Switch 2
to swap out these numbers and now this will go blocking
and this will go forwarding, so let's verify that.
Let's go back to Ether Switch 2 and make sure that
1/15 is now in the forwarding stage.
And there we can see, the lower number port
is now blocking, the higher number port is now forwarding.
And that concludes this CCNA level switching lab.
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