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Okay so let's see if a trunk is formed between switch 1 and switch 2

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and if a trunk is formed between switch 2 and switch 3. This is the capture between switch 1 and

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switch 2.

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I'm gonna do a search for DTP, so I've named the switches to make it easy to know which switch

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is sending DTP messages.

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Cisco switch 1, Cisco switch 2.

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So again this is the link between these two switches.

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If we look at dynamic trunk protocol

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notice operating administer of mode is access order, operating administer of ISL negotiated. So it looks

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like they may negotiate to use ISL trunk status is access auto and what I'll do is just make this bigger

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so we can see the full Wireshark capture.

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Notice trunk operating status as access, trunk administrative status is auto.

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In other words auto has been used on that switch.

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Now this is switch 1 and I could show you that by looking at the console of the switch. So console

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of switch 1,

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show interface trunk firstly shows us that no trunks are formed

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but notice show interface gigabit 00 switch port.

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What you'll notice here is this command shows the interface, interface is a switch port administer of

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mode is dynamic auto, operational mode is static access, in the Wireshark capture we see operating status

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as access, administrative mode is auto, so administer of mode is dynamic auto over there that's what

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we see here static access is the operational mode so operating mode is access.

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So at this point there's no trunking on that port, the port is acting as an access VLAN in VLAN 1 as

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you can see over there.

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Negotiation of trunking is on but a trunk is not formed

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and as I go through the messages I mean you must be careful looking at the very first messages go and

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look at the later messages to see if the state changed, notice store access auto trunk type is ISL

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negotiated they would negotiate to use ISL if one of the sides negotiated. Switch 2 is also set to

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access auto, so the point is notice over here DTP operating administerr of mode, access auto operating

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administer of ISL negotiated both these switches are using DTP. I'm searching for DTP messages and we

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can see DTP messages but they're set to mode auto, mode auto or dynamic auto as we can see here means that

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that side will not negotiate to form a trunk the other side would have to initiate trunking. Here switch

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2 show interface gigabit 00 switch port.

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Notice dynamic auto that is the administrative mode operational mode is static access, so they both configured

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to use dynamic auto because neither of them initiate a connection to the other side they both passively

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waiting basically for the other side to initiate trunking.

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Trunking is not formed, so there is no trunk formed between switch 1 and switch 2. I won't use the

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show trunk command and switch to yet because we wanna see what happens on the right end side.

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But notice here show interface trunk, no trunk is formed on switch 1. Okay, so I kind of hinted at it.

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What do you think is gonna happen between switch 2 and switch 3. Here's the Wireshark capture between

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switch 2 and switch 3, I'll filter for DTP once again. Here's switch 3 messages.

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Notice the difference dynamic trunk protocol, access desirable, operating administrative mode ISL 

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dot1q.

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So that's different to what we saw on the other side.

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00:04:38,750 --> 00:04:41,340
If we look at switch 2 on this link

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00:04:41,340 --> 00:04:50,200
notice trunk auto 802.1Q negotiated, so a little bit different, notice

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00:04:50,200 --> 00:04:51,770
switch 2, switch 3.

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00:04:51,790 --> 00:04:57,140
One side is using auto the other side is using desirable.

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00:04:57,160 --> 00:05:06,660
So if I go right down to the end this is switch 2, trunk status is type trunk, trunk

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value is trunk, auto trunk operating status is a trunk.

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00:05:12,910 --> 00:05:19,640
Now if we go back to the other switches, so this is switch 2 talking to switch 1 and we look at the

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00:05:19,640 --> 00:05:27,800
trunk status, notice trunk status is access and I don't know if that's confusing so let me show you on the

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00:05:27,800 --> 00:05:28,820
diagram.

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00:05:28,890 --> 00:05:35,780
So on the left en side switch 2 to switch 1 this is the capture between switch 1, switch 2 notice

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00:05:36,560 --> 00:05:38,910
operating status is access.

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00:05:39,290 --> 00:05:43,100
This is an access port, but on this side

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00:05:43,190 --> 00:05:51,450
notice trunk operating status is trunk, that side's access right end side is a trunk.

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00:05:51,520 --> 00:05:58,170
It's become a trunk, trunk administrative status is auto the side it's also auto

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00:05:58,180 --> 00:06:07,420
but because switch 3 initiated the trunk, notice it says desirable here a trunk was formed between

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00:06:07,420 --> 00:06:16,740
switch 2 and switch 3. So because switch 3 initiated the trunk because it's set to desirable

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a trunk is formed we can see that once again by looking at the console of the switches.

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So on switch 3 show interface gigabit 00 switch port.

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Notice command shows administer mode dynamic desirable, encapsulation used is dot1q, operational mode is

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trunk, it's operating as a trunk using dot1q. Show interface trunk we can see that, encapsulation

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is dot1q, trunking is enabled and if we have a look on the other side it's a little bit different here.

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00:06:59,810 --> 00:07:09,360
Show interface trunk, on this side it's negotiated to use dot1q and trunking is enabled, whereas this 

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00:07:09,370 --> 00:07:15,890
side encapsulation is dot1q and the reason for that is I explicitly configured the encapsulation

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00:07:15,890 --> 00:07:24,920
as dot1q and set it to dynamic desirable. I wanted to show you a trunk that was negotiated with DTP

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00:07:25,640 --> 00:07:33,170
and using dot1q rather than ISL if I had not put this in the switches would have actually negotiated

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00:07:33,380 --> 00:07:41,480
to form an ISL trunk. these switches will use ISL first and then dot1q. So I wanted to explicitly

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00:07:41,480 --> 00:07:47,090
force it to use dot1q. So they will try and use ISL but if they can't then they'll use dot1q.

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00:07:47,660 --> 00:07:55,000
But again I've overridden that or I forced the switch to use dot1q whereas this side notice it could

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00:07:54,990 --> 00:08:04,720
have used ISL or dot1q. So again on switch 3 if we got to go up here I noticed trunk desirable

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00:08:05,110 --> 00:08:15,100
dot1q, dot1q whereas on switch2 trunk auto dot1q negotiated. So notice desirable dot

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1q, dot1q trunk auto dot1q negotiated, one side was forced to use dot1q the other side negotiated

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00:08:23,860 --> 00:08:30,120
to use dot1q. Okay, so, in summary, left end side doesn't become a trunk because both sides are set

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00:08:30,120 --> 00:08:37,770
to auto right hand side does become a trunk because one side was set to dynamic desirable as we can

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00:08:37,770 --> 00:08:46,050
see over there and uses dot1q on this side show interface trunk the only trunk that was negotiated

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is on gigabit 01. Okay how are you doing?

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Are you able to answer these questions do you enjoy this? Please give me a feedback.

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Do you enjoy these kind of quiz questions.