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- In the last video I introduced you

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to the concept of VLAN trunking.

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We took a look at the VLAN tags,

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the dot1Q tags in our sniffer trace,

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and initially the way
we configured our Trunk

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was using the command
switchport mode trunk.

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Changing it from an access
mode to a trunking mode.

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Now when you use switchport mode trunk,

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what you're telling the
interface essentially is,

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I don't care what you're connected to

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on the other end of the cable,

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you will do trunking.

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So theoretically, if you messed up,

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you might accidentally
configure that command

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on a port leading to
a host, like a laptop,

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and now you have a problem,

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because that switch
would be sending frames

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down to that laptop.

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Some of those frames having tags,

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and the laptops don't
understand 802.1Q tags,

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they don't know what to do with it.

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So your Ethernet NIC card on your laptop

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would be getting all these sorts of frames

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it would have to be throwing away,

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because it didn't know what trunking was.

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But when you type switchport mode trunk

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on the switch interface, it has no choice.

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It is doing VLAN trunking.

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So another issue or
potential problem with that,

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from a more practical standpoint,

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is we talked earlier, I don't
remember how many videos ago,

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but I mentioned how interfaces,

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I think this was when I first introduced

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the concept of switches and bridges

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and how each port is its own
separate collision domain.

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And I mentioned that the
way that was possible

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was that switches and bridges had memory,

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so that if two frames come in

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at the same time on one switch,

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they're stored in separate
sections of memory,

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so they don't run into each other.

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And that's what I've drawn right here.

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So this is one cable,

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let's just say this is, I don't know,

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FastEthernet0/2 on one side,

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and we'll just keep it the
same on the other side,

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just to make it easy.

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And so on Switch-1, this
port, FastEthernet0/2,

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has some transmit buffer,

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in other words, if a frame
comes in somewhere else

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and it has to go out FastEthernet0/2,

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it'll go into the transmit buffer first,

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wait for like a picosecond,
really really fast,

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and then go out on the wire.

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And the receive buffer
is just the opposite.

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If a frame comes in, it goes
into the receive buffer,

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then gets looked up and switched out.

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So that's how these,
this is memory, right?

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Computer memory.

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And that's what this is used for.

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Well let's say that on
both sides of this link,

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I statically configured these
two interfaces to be trunks.

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So I did config t,

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interface fast0/2,

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switchport mode trunk.

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Okay, it's chugging along, it's just fine,

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it's operating as a trunk.

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But let's say, and this doesn't
happen a lot thank goodness,

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but every once in a while,
memory gets corrupted.

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Maybe there was a huge lightning storm

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and there were some power surges,

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or you know, whatever happened,

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but some memory gets corrupted.

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So let's say that the
receive buffer on Switch-2

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becomes bad, and what I
mean by becomes bad is,

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either as electrical energy comes in

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it can no longer store the
ones and zeroes in here,

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or as it's trying to
store the ones and zeroes

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it's corrupting them in some way

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and they're getting all jumbled.

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So basically, stuff that's
coming in off the wire,

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going in this direction, is useless,

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because it's just dying
as it comes right here,

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the receive buffer.

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Well Switch-1 doesn't know that.

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He has no idea that's a problem.

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So as far as Switch-1's concerned,

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he's happily chugging
away, receiving frames,

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and then putting them
in the transmit buffer,

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and then sending them across here,

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and then they're just dying on Switch-2,

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'cause he's got no place to store them,

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or they're being corrupted.

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So it might be a better
idea if you said, well,

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and let's say, to make
things even more practical,

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as an example,

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let's say that there were,
for redundancy's sake,

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two cables.

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You said hey, you know
what, if one trunk fails,

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I wanna have another
trunk pick up the slack.

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So I'm gonna configure
this as a trunk as well.

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Now when you learn about Spanning-Tree,

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at the ICND2 level,

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you'll learn that Spanning-Tree
doesn't like this.

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When Spanning-Tree sees
two redundant connections

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between switches, it says,

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hm, there's a problem here.

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Because if a broadcast
comes in on Switch-1,

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it could end up just circling

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around these two interfaces forever,

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circling and circling and circling,

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and so Spanning-Tree says,
nope, we're gonna prevent that.

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We're basically gonna take
down one of these ports.

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It doesn't really do that, it's called

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putting it in blocking state.

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But it says, I'm gonna block this port.

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So on this link up here,

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I'm not gonna allow any
data to go back and forth.

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We're just gonna put it
on this link down here.

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So the idea is,

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if this bottom link fails,

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if I actually yanked out
the cable or shut it down,

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then Spanning-Tree would say,

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okay I'm now going to
unblock this top link.

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Now traffic can go across this top link.

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So I'm giving you sort of
a preview of Spanning-Tree

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that you'll learn at the ICND2 level.

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But now we have a problem here.

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I'm gonna put this up here.

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BLOCKED by Spanning-Tree.

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So here we have an issue, because Switch-1

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doesn't know that the bottom link is bad.

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From his perspective, this link is up up,

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there's no issues with it,

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so he's still taking all the traffic,

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and because Spanning-Tree has told him

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the bottom link is what
you should be using,

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that's forwarding, forward
your traffic that way,

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he's still putting it on that trunk,

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and it's dying as it gets to Switch-2.

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He has no idea that this trunk is bad.

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So what we could do instead

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is do what's called dynamic trunking.

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With dynamic trunking, these two switches

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have to initially exchange
some special messages

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across this bottom link, and the top link,

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if I want the top link
to be trunking as well,

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special Cisco proprietary messages

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that say, I'd like to trunk
with you, is that okay?

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Now if that message exchange works,

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then we can bring up the
trunk, but guess what?

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As long as that trunk is up,

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those special messages
are still exchanged.

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Hey, you still good?

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We still good for trunking?

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Yeah yeah, we're alright, we're alright.

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So in this particular
case, that would fail.

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If we used dynamic trunking here,

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the special messages
that Switch-1 was sending

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and saying, hey I'd like to
trunk with you, is that okay?

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All of a sudden Switch-2

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would not be getting
those messages anymore.

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Why is that?

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Well because they would be coming in,

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but they'd have to be stored
in the receive buffer.

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Anything that comes in on FastEthernet0/2

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has to be stored in the receive buffer,

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but our receive buffer is bad.

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So Switch-2 would say, um, we
initially agreed on trunking,

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and you're supposed to be

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sending me these messages periodically,

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and I'm not hearing from you.

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It's not that Switch-1's not sending them.

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Switch-1 is sending them.

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But when Switch-2 receives them,

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they're getting all messed up.

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So from Switch-2's perspective,

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he's saying, something's wrong here.

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And when that exchange of
that messaging protocol fails,

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then Switch-2 says, okay I'm
no longer gonna trunk with you.

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Switch-2 would actually
send a message back saying,

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trunking failed, no longer trunking,

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and this would revert
back to an access port.

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That's what dynamic trunking can do.

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It exchanges these special
Cisco proprietary messages

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to bring up the trunk,

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and then as the trunk is up,

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these messages have to
periodically still be exchanged

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as keep alives, as hellos,

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and if something happens
where that exchange stops,

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now the trunk reverts back to access mode.

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Now you might be wondering,
"Well wait a second.

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"If it reverts back to access mode,

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"that means it was carrying all the VLANs,

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"all the VLANs that were
allowed, that were configured,

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"and now it's carrying only one VLAN.

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"Which VLAN is it gonna carry?"

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That all depends on what
you've configured here

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in your switchport access vlan command.

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So if I had switchport access vlan 4,

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while it's operating as a trunk,

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this command would do nothing,

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'cause it's not an access port.

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But if the trunk failed
and it had to revert

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back to being an access port,

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now it would be an access port in vlan 4.

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So how do we get this
dynamic trunking going on?

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It's not the switchport mode trunk,

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'cause switchport mode trunk
forces it to be a trunk.

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And that's what we're gonna
talk about right here.

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So Cisco has, this proprietary language

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that's going back and
forth between the switches

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is called the Dynamic Trunking Protocol.

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I wish I could capture
it in a sniffer trace,

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but unfortunately the switches
available to me in GNS3,

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don't support this.

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So I can't do that.

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But you could google it.

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I would recommend, google
Cisco DTP sniffer capture,

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and you could probably find

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a picture of what that looks like.

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But the Dynamic Trunking
Protocol is that special language

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that says, okay, can you
trunk with me, are we good?

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And then if the trunk comes up,

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they still exchange DTP back and forth

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to maintain the trunk, to keep it up.

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Now clearly, Cisco proprietary.

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So if I have a Cisco switch
connected to a non-Cisco switch,

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you don't have a choice.

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Then you'll have to do
switchport mode trunk

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on the Cisco switch and force it.

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Another key thing.

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DTP only understood by Cisco switches,

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not understood by routers.

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Remember in the last
lab, or the last video,

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when I introduced the
concept of VLAN trunking.

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Still have the GNS3 up here.

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The way I did it was

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I had also trunking going
from the switch to the router.

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This cannot be a dynamic trunk,

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because the router does not understand

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the Dynamic Trunking Protocol.

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So on this interface, on this switch,

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I would have to do switchport mode trunk,

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even though I'm connecting
to a Cisco router.

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'Cause the Cisco router
doesn't understand DTP.

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But from one Cisco switch
to another Cisco switch,

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I could use DTP.

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Now it's optional, you
don't have to use it,

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but you know on this
whiteboard right here,

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this is one reason why you might want to.

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If you think there's a possibility

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where the interface might still be up,

254
00:11:20,369 --> 00:11:23,614
but it might not actually be
reachable on the other side.

255
00:11:23,614 --> 00:11:25,624
Something is preventing communication,

256
00:11:25,624 --> 00:11:28,571
bi-directional communication,
back and forth,

257
00:11:28,571 --> 00:11:30,907
but the interface is still physically up.

258
00:11:30,907 --> 00:11:32,396
That would be a good use case

259
00:11:32,396 --> 00:11:36,246
where dynamic trunking could detect that.

260
00:11:36,246 --> 00:11:40,955
So when you're configuring
trunking you have three options.

261
00:11:40,955 --> 00:11:45,122
So option number one is
just switchport mode trunk,

262
00:11:46,252 --> 00:11:48,614
and that's what they're showing you here

263
00:11:48,614 --> 00:11:51,115
as using the keyword of on.

264
00:11:51,115 --> 00:11:53,330
There really is no keyword of on,

265
00:11:53,330 --> 00:11:56,497
it's just switchport mode trunk is on,

266
00:11:57,984 --> 00:11:59,851
so I'll just write that here,

267
00:11:59,851 --> 00:12:01,684
switchport mode trunk.

268
00:12:06,721 --> 00:12:09,138
Okay, now auto and desirable,

269
00:12:10,441 --> 00:12:12,747
these here, to use these,

270
00:12:12,747 --> 00:12:15,342
you have to go into a different mode.

271
00:12:15,342 --> 00:12:17,759
And to use auto or desirable,

272
00:12:19,414 --> 00:12:22,805
you use switchport mode dynamic,

273
00:12:22,805 --> 00:12:26,023
because we're doing dynamic trunking.

274
00:12:26,023 --> 00:12:29,390
And then you get to
choose, desirable or auto.

275
00:12:29,390 --> 00:12:31,223
Now almost a guarantee

276
00:12:32,509 --> 00:12:35,685
that whether you take the
ICND1, the ICND2, you know,

277
00:12:35,685 --> 00:12:38,061
whatever you take, I
can almost promise you

278
00:12:38,061 --> 00:12:39,663
that you're gonna get a question

279
00:12:39,663 --> 00:12:41,450
testing your understanding of,

280
00:12:41,450 --> 00:12:42,650
do you know what the differences are

281
00:12:42,650 --> 00:12:46,483
between dynamic desirable
versus dynamic auto?

282
00:12:47,523 --> 00:12:49,624
So let me tell you.

283
00:12:49,624 --> 00:12:52,576
So when you say switchport
mode dynamic desirable,

284
00:12:52,576 --> 00:12:54,379
as soon as you hit Enter,

285
00:12:54,379 --> 00:12:56,687
if that interface is up and connected,

286
00:12:56,687 --> 00:12:58,666
it starts sending out DTP messages,

287
00:12:58,666 --> 00:13:00,265
saying hey, I'd like to trunk with you,

288
00:13:00,265 --> 00:13:01,754
can you do that, huh huh huh huh?

289
00:13:01,754 --> 00:13:02,970
You wanna trunk with me?

290
00:13:02,970 --> 00:13:04,269
I would desire that.

291
00:13:04,269 --> 00:13:06,324
That's desirable.

292
00:13:06,324 --> 00:13:10,210
So if you do switchport mode dynamic auto,

293
00:13:10,210 --> 00:13:12,304
he's like sitting back with
his hands folded saying,

294
00:13:12,304 --> 00:13:14,727
okay, you could talk to me,

295
00:13:14,727 --> 00:13:17,075
but I'm not gonna talk to you.

296
00:13:17,075 --> 00:13:18,650
In other words, you've gotta initiate.

297
00:13:18,650 --> 00:13:19,981
I'm just gonna sit here and listen,

298
00:13:19,981 --> 00:13:23,314
I'll talk to you if you talk to me.

299
00:13:23,314 --> 00:13:25,790
That's auto, so auto is passive.

300
00:13:25,790 --> 00:13:29,550
It does not initiate the conversation.

301
00:13:29,550 --> 00:13:32,132
So if we think about it, and by the way,

302
00:13:32,132 --> 00:13:34,965
when you do switchport mode trunk,

303
00:13:39,538 --> 00:13:41,871
as we saw in the last video,

304
00:13:42,752 --> 00:13:45,205
please forgive my bad handwriting here,

305
00:13:45,205 --> 00:13:47,038
switchport mode trunk,

306
00:13:53,128 --> 00:13:54,128
DTP is sent.

307
00:14:03,733 --> 00:14:06,592
So even when you do switchport mode trunk,

308
00:14:06,592 --> 00:14:09,411
the side that's configured
as switchport mode trunk,

309
00:14:09,411 --> 00:14:11,451
he's not doing anything dynamically,

310
00:14:11,451 --> 00:14:13,882
you've just told him, you will be a trunk,

311
00:14:13,882 --> 00:14:15,554
you have no choice.

312
00:14:15,554 --> 00:14:18,612
But he is sending out DTP as a courtesy,

313
00:14:18,612 --> 00:14:23,225
so that the other side could
be desirable if it wants to.

314
00:14:23,225 --> 00:14:25,018
So that being the case,
we have to think about,

315
00:14:25,018 --> 00:14:28,896
what combinations of keywords
will work to form a trunk?

316
00:14:28,896 --> 00:14:30,313
Which ones won't?

317
00:14:31,304 --> 00:14:35,110
Pretty much every combination
of these keywords will work

318
00:14:35,110 --> 00:14:37,777
to bring up a trunk, other than,

319
00:14:40,396 --> 00:14:42,646
if I have auto on one side,

320
00:14:43,729 --> 00:14:45,562
and auto on the other.

321
00:14:47,393 --> 00:14:48,692
If both sides are auto,

322
00:14:48,692 --> 00:14:51,490
neither side is initiating
the conversation.

323
00:14:51,490 --> 00:14:53,530
They're both waiting for
the other side to talk,

324
00:14:53,530 --> 00:14:55,431
and the other side's not gonna talk.

325
00:14:55,431 --> 00:14:56,784
So in that particular case,

326
00:14:56,784 --> 00:14:59,441
this link will remain as an access port

327
00:14:59,441 --> 00:15:02,382
in whatever VLAN it's configured to be in.

328
00:15:02,382 --> 00:15:04,098
It will not do trunking.

329
00:15:04,098 --> 00:15:06,996
Somebody has to initiate the conversation.

330
00:15:06,996 --> 00:15:08,714
So that could be a desirable,

331
00:15:08,714 --> 00:15:10,971
or that could be switchport mode trunk,

332
00:15:10,971 --> 00:15:13,363
but one side has to initiate.

333
00:15:13,363 --> 00:15:15,702
In a lot of Cisco documents,
they actually recommend

334
00:15:15,702 --> 00:15:18,785
that you set both sides to desirable.

335
00:15:23,555 --> 00:15:25,533
That's what they typically suggest,

336
00:15:25,533 --> 00:15:29,562
is you do switchport mode
dynamic desirable on both sides.

337
00:15:29,562 --> 00:15:30,789
But I'm just showing you here

338
00:15:30,789 --> 00:15:32,924
there's a lot of other
combinations that could work.

339
00:15:32,924 --> 00:15:35,714
I could have switchport
remote trunk, mode auto,

340
00:15:35,714 --> 00:15:39,150
switchport mode dynamic
desirable, dynamic auto,

341
00:15:39,150 --> 00:15:41,487
dynamic desirable, dynamic desirable,

342
00:15:41,487 --> 00:15:44,191
you know as long as someone
is initiating the exchange,

343
00:15:44,191 --> 00:15:48,108
the other dynamic side
can pick it up and talk.

344
00:15:53,348 --> 00:15:57,431
And we verify it, oh, and
switchport nonegotiate.

345
00:15:59,108 --> 00:16:01,033
Really the only time I
can think of where this

346
00:16:01,033 --> 00:16:04,290
might be something you wanna do is,

347
00:16:04,290 --> 00:16:06,923
let's say that you have an
interface on your switch

348
00:16:06,923 --> 00:16:11,090
connected to a device that
does not understand DTP,

349
00:16:12,520 --> 00:16:14,210
and you just say to yourself, well,

350
00:16:14,210 --> 00:16:17,732
there's really no point in
putting DTP on that wire.

351
00:16:17,732 --> 00:16:20,203
I mean it takes up tiny
little bit of bandwidth,

352
00:16:20,203 --> 00:16:22,254
I mean it doesn't go out that frequently,

353
00:16:22,254 --> 00:16:23,699
but if you say to yourself, you know what?

354
00:16:23,699 --> 00:16:27,008
I don't even want it going
out at all, there's no point.

355
00:16:27,008 --> 00:16:29,458
Well you could say switchport mode trunk,

356
00:16:29,458 --> 00:16:30,785
force it to trunk,

357
00:16:30,785 --> 00:16:35,048
and then on top of that,
say switchport nonegotiate.

358
00:16:35,048 --> 00:16:36,849
That means don't send any DTP at all,

359
00:16:36,849 --> 00:16:39,266
just turn off DTP processing.

360
00:16:41,435 --> 00:16:43,692
And verifying it is the exact same way

361
00:16:43,692 --> 00:16:45,948
that you verify it with
the other show commands

362
00:16:45,948 --> 00:16:47,698
that we've looked at.

363
00:16:48,757 --> 00:16:51,640
So let's just do a quick lab on that,

364
00:16:51,640 --> 00:16:54,473
so you can see what it looks like.

365
00:16:55,862 --> 00:16:59,862
So for this, I am going
to log into INE's Racks.

366
00:17:01,613 --> 00:17:05,345
And because I've used
our racks so frequently,

367
00:17:05,345 --> 00:17:08,345
I know what the topology looks like.

368
00:17:09,324 --> 00:17:11,854
But I'll just draw it
here real quickly for you.

369
00:17:11,854 --> 00:17:15,687
I know that Switch-1 is
connected to Switch-2,

370
00:17:20,142 --> 00:17:24,473
via FastEthernet0/10 on both sides.

371
00:17:24,473 --> 00:17:27,190
So that's what I'll be configuring.

372
00:17:27,190 --> 00:17:29,733
Now one other thing
you should be aware of.

373
00:17:29,733 --> 00:17:31,954
You might be wondering,
well when it comes to modes,

374
00:17:31,954 --> 00:17:35,178
so so far we've talked about
switchport mode access,

375
00:17:35,178 --> 00:17:37,393
switchport mode trunk,

376
00:17:37,393 --> 00:17:39,986
and switchport mode dynamic,

377
00:17:39,986 --> 00:17:42,853
with either auto or
desirable as the keywords.

378
00:17:42,853 --> 00:17:45,442
You might be wondering,
what's the default mode?

379
00:17:45,442 --> 00:17:48,021
When I just wipe out the configuration

380
00:17:48,021 --> 00:17:49,512
and start from scratch,

381
00:17:49,512 --> 00:17:51,645
or I take a brand new
switch and I turn it on,

382
00:17:51,645 --> 00:17:54,616
which of those modes is it by default?

383
00:17:54,616 --> 00:17:57,551
Pretty much all Cisco
switches, by default,

384
00:17:57,551 --> 00:17:59,884
are switchport mode dynamic.

385
00:18:01,429 --> 00:18:02,954
You might be saying, "Well wait a second,

386
00:18:02,954 --> 00:18:04,184
"you can't be done yet,

387
00:18:04,184 --> 00:18:07,213
"it's either dynamic auto
or dynamic desirable.

388
00:18:07,213 --> 00:18:08,459
"Which one is it?"

389
00:18:08,459 --> 00:18:10,792
That depends on the model of switch.

390
00:18:10,792 --> 00:18:15,240
Some switches are switchport
mode dynamic auto by default.

391
00:18:15,240 --> 00:18:16,558
Most of them are.

392
00:18:16,558 --> 00:18:21,240
But some switches are switchport
mode dynamic desirable,

393
00:18:21,240 --> 00:18:22,695
by default.

394
00:18:22,695 --> 00:18:25,509
And that can sometimes
throw people for a loop,

395
00:18:25,509 --> 00:18:26,771
because they say hey,

396
00:18:26,771 --> 00:18:29,367
I just connected these switches
back to back with cables

397
00:18:29,367 --> 00:18:31,112
and these trunks just came up.

398
00:18:31,112 --> 00:18:33,629
I didn't mean for that to
happen, it just happened.

399
00:18:33,629 --> 00:18:35,217
Well that's because at least one of them

400
00:18:35,217 --> 00:18:37,797
was dynamic desirable by default,

401
00:18:37,797 --> 00:18:39,172
and as soon as you connected the cable,

402
00:18:39,172 --> 00:18:43,464
it starts sending DTP,
without you doing anything.

403
00:18:43,464 --> 00:18:45,964
Let's see what these ones are.

404
00:18:46,811 --> 00:18:50,144
I know these are Catalyst 3560 switches.

405
00:18:51,963 --> 00:18:54,643
So one way you can tell is,

406
00:18:54,643 --> 00:18:56,376
first of all, let me take
a look at the interface

407
00:18:56,376 --> 00:18:58,710
and see if it has any
existing configuration on it,

408
00:18:58,710 --> 00:18:59,543
no it doesn't.

409
00:18:59,543 --> 00:19:03,314
Okay so, this interface
doesn't have any configuration.

410
00:19:03,314 --> 00:19:07,481
So I can type show interface
fast0/10 switchport.

411
00:19:12,834 --> 00:19:16,706
And there we go, dynamic
auto, it's dynamic auto.

412
00:19:16,706 --> 00:19:18,873
So that's why it will not,

413
00:19:20,752 --> 00:19:23,139
form a trunk until I tell it to.

414
00:19:23,139 --> 00:19:27,056
Okay, so I'll go into int fast 0/10.

415
00:19:29,100 --> 00:19:32,767
And I'll say, let's
just shut it down first.

416
00:19:34,604 --> 00:19:36,908
I typically recommend
that if you're changing

417
00:19:36,908 --> 00:19:39,647
the physical layer or data link layer

418
00:19:39,647 --> 00:19:41,734
characteristics of an interface,

419
00:19:41,734 --> 00:19:44,239
it's a good idea to shut it down first,

420
00:19:44,239 --> 00:19:46,502
make your layer 1 or layer 2 changes,

421
00:19:46,502 --> 00:19:49,353
and then do a no shutdown afterwards.

422
00:19:49,353 --> 00:19:51,777
Sometimes when you try to
change those types of things

423
00:19:51,777 --> 00:19:55,460
while the interface is up,
weird things can happen.

424
00:19:55,460 --> 00:19:57,793
So let's just do switchport,

425
00:20:00,753 --> 00:20:03,586
switchport mode dynamic desirable.

426
00:20:08,976 --> 00:20:13,143
While we're at it, switchport
trunk encapsulation dot1q.

427
00:20:18,799 --> 00:20:19,716
Switchport.

428
00:20:22,600 --> 00:20:24,100
No shutdown.

429
00:20:24,100 --> 00:20:26,991
Now before I do a no shutdown,

430
00:20:26,991 --> 00:20:29,278
I don't know, 'cause I haven't looked yet,

431
00:20:29,278 --> 00:20:32,283
what the state is of this interface.

432
00:20:32,283 --> 00:20:35,798
If this interface is ready to go,

433
00:20:35,798 --> 00:20:39,389
if it is not administratively
disabled, if it's ready to go,

434
00:20:39,389 --> 00:20:42,328
and if it is in the default auto state,

435
00:20:42,328 --> 00:20:44,614
the default state of dynamic auto,

436
00:20:44,614 --> 00:20:46,948
the moment I do no shut on here,

437
00:20:46,948 --> 00:20:50,684
I just configured him
to be dynamic desirable.

438
00:20:50,684 --> 00:20:52,396
So the moment I bring him up,

439
00:20:52,396 --> 00:20:56,563
he should start sending DTP,
the Dynamic Trunking Protocol.

440
00:20:58,167 --> 00:21:00,877
And if this other side
comes up, and he's auto,

441
00:21:00,877 --> 00:21:02,823
the trunk should come up.

442
00:21:02,823 --> 00:21:05,649
I shouldn't have to do
anything on Switch-2.

443
00:21:05,649 --> 00:21:08,649
So let's just see what happens here.

444
00:21:10,772 --> 00:21:13,015
Interface changed state to down.

445
00:21:13,015 --> 00:21:16,186
Okay so that's telling
me, remember what we said?

446
00:21:16,186 --> 00:21:20,992
Remember what I said when I
said show IP interface brief.

447
00:21:20,992 --> 00:21:22,717
And it says our troubleshooting step,

448
00:21:22,717 --> 00:21:24,702
if I have an interface that I know

449
00:21:24,702 --> 00:21:26,831
is connected to something
and it's down down,

450
00:21:26,831 --> 00:21:27,942
what did I say?

451
00:21:27,942 --> 00:21:31,453
I said well, maybe there's
no cable connected at all.

452
00:21:31,453 --> 00:21:33,974
Maybe there is a cable, but
the other end of that cable

453
00:21:33,974 --> 00:21:36,697
is not connected to anything.

454
00:21:36,697 --> 00:21:38,956
Or the other end of that cable
is connected to something,

455
00:21:38,956 --> 00:21:41,833
but the other side is
administratively down.

456
00:21:41,833 --> 00:21:43,333
Let's take a look.

457
00:21:45,202 --> 00:21:49,149
Show run interface FastEthernet0/10.

458
00:21:49,149 --> 00:21:50,156
See?

459
00:21:50,156 --> 00:21:51,406
It is shutdown.

460
00:21:58,231 --> 00:22:01,807
Okay, changed state to up, that's good.

461
00:22:01,807 --> 00:22:03,756
And if this becomes a trunk,

462
00:22:03,756 --> 00:22:07,923
I should get some messaging
about that, I would think.

463
00:22:08,785 --> 00:22:12,972
Changed state to up, so
this is the data link layer,

464
00:22:12,972 --> 00:22:16,069
the line protocol indicates layer 2.

465
00:22:16,069 --> 00:22:18,239
This here indicates the physical layer.

466
00:22:18,239 --> 00:22:20,236
So far I haven't gotten any messaging

467
00:22:20,236 --> 00:22:22,796
about it being a trunk.

468
00:22:22,796 --> 00:22:24,093
What is the command I can type

469
00:22:24,093 --> 00:22:28,093
to see if I have any VLAN
trunks on this switch?

470
00:22:30,416 --> 00:22:33,666
That command was show interfaces trunk.

471
00:22:35,055 --> 00:22:36,146
And it is.

472
00:22:36,146 --> 00:22:37,600
So I didn't get any Cis log message,

473
00:22:37,600 --> 00:22:38,457
I didn't see any messages about it,

474
00:22:38,457 --> 00:22:39,290
but here it is.

475
00:22:39,290 --> 00:22:41,528
It says it's in auto mode,

476
00:22:41,528 --> 00:22:44,752
n means it dynamically negotiated,

477
00:22:44,752 --> 00:22:49,005
that's what the n is for,
it negotiated 802.1q.

478
00:22:49,005 --> 00:22:51,552
And here it says native VLAN is 1.

479
00:22:51,552 --> 00:22:53,243
So this says it's in dynamic auto,

480
00:22:53,243 --> 00:22:55,025
let's take a look at Switch-1,

481
00:22:55,025 --> 00:22:58,192
who I configured as dynamic desirable.

482
00:22:59,151 --> 00:23:00,984
Show interfaces trunk.

483
00:23:03,105 --> 00:23:03,973
See?

484
00:23:03,973 --> 00:23:05,973
His mode says desirable.

485
00:23:07,724 --> 00:23:10,095
And that concludes this section

486
00:23:10,095 --> 00:23:13,178
on Cisco's Dynamic Trunking Protocol.