Okay so let's see if a trunk is formed between switch 1 and switch 2
and if a trunk is formed between switch 2 and switch 3. This is the capture between switch 1 and
switch 2.
I'm gonna do a search for DTP, so I've named the switches to make it easy to know which switch
is sending DTP messages.
Cisco switch 1, Cisco switch 2.
So again this is the link between these two switches.
If we look at dynamic trunk protocol
notice operating administer of mode is access order, operating administer of ISL negotiated. So it looks
like they may negotiate to use ISL trunk status is access auto and what I'll do is just make this bigger
so we can see the full Wireshark capture.
Notice trunk operating status as access, trunk administrative status is auto.
In other words auto has been used on that switch.
Now this is switch 1 and I could show you that by looking at the console of the switch. So console
of switch 1,
show interface trunk firstly shows us that no trunks are formed
but notice show interface gigabit 00 switch port.
What you'll notice here is this command shows the interface, interface is a switch port administer of
mode is dynamic auto, operational mode is static access, in the Wireshark capture we see operating status
as access, administrative mode is auto, so administer of mode is dynamic auto over there that's what
we see here static access is the operational mode so operating mode is access.
So at this point there's no trunking on that port, the port is acting as an access VLAN in VLAN 1 as
you can see over there.
Negotiation of trunking is on but a trunk is not formed
and as I go through the messages I mean you must be careful looking at the very first messages go and
look at the later messages to see if the state changed, notice store access auto trunk type is ISL
negotiated they would negotiate to use ISL if one of the sides negotiated. Switch 2 is also set to
access auto, so the point is notice over here DTP operating administerr of mode, access auto operating
administer of ISL negotiated both these switches are using DTP. I'm searching for DTP messages and we
can see DTP messages but they're set to mode auto, mode auto or dynamic auto as we can see here means that
that side will not negotiate to form a trunk the other side would have to initiate trunking. Here switch
2 show interface gigabit 00 switch port.
Notice dynamic auto that is the administrative mode operational mode is static access, so they both configured
to use dynamic auto because neither of them initiate a connection to the other side they both passively
waiting basically for the other side to initiate trunking.
Trunking is not formed, so there is no trunk formed between switch 1 and switch 2. I won't use the
show trunk command and switch to yet because we wanna see what happens on the right end side.
But notice here show interface trunk, no trunk is formed on switch 1. Okay, so I kind of hinted at it.
What do you think is gonna happen between switch 2 and switch 3. Here's the Wireshark capture between
switch 2 and switch 3, I'll filter for DTP once again. Here's switch 3 messages.
Notice the difference dynamic trunk protocol, access desirable, operating administrative mode ISL
dot1q.
So that's different to what we saw on the other side.
If we look at switch 2 on this link
notice trunk auto 802.1Q negotiated, so a little bit different, notice
switch 2, switch 3.
One side is using auto the other side is using desirable.
So if I go right down to the end this is switch 2, trunk status is type trunk, trunk
value is trunk, auto trunk operating status is a trunk.
Now if we go back to the other switches, so this is switch 2 talking to switch 1 and we look at the
trunk status, notice trunk status is access and I don't know if that's confusing so let me show you on the
diagram.
So on the left en side switch 2 to switch 1 this is the capture between switch 1, switch 2 notice
operating status is access.
This is an access port, but on this side
notice trunk operating status is trunk, that side's access right end side is a trunk.
It's become a trunk, trunk administrative status is auto the side it's also auto
but because switch 3 initiated the trunk, notice it says desirable here a trunk was formed between
switch 2 and switch 3. So because switch 3 initiated the trunk because it's set to desirable
a trunk is formed we can see that once again by looking at the console of the switches.
So on switch 3 show interface gigabit 00 switch port.
Notice command shows administer mode dynamic desirable, encapsulation used is dot1q, operational mode is
trunk, it's operating as a trunk using dot1q. Show interface trunk we can see that, encapsulation
is dot1q, trunking is enabled and if we have a look on the other side it's a little bit different here.
Show interface trunk, on this side it's negotiated to use dot1q and trunking is enabled, whereas this
side encapsulation is dot1q and the reason for that is I explicitly configured the encapsulation
as dot1q and set it to dynamic desirable. I wanted to show you a trunk that was negotiated with DTP
and using dot1q rather than ISL if I had not put this in the switches would have actually negotiated
to form an ISL trunk. these switches will use ISL first and then dot1q. So I wanted to explicitly
force it to use dot1q. So they will try and use ISL but if they can't then they'll use dot1q.
But again I've overridden that or I forced the switch to use dot1q whereas this side notice it could
have used ISL or dot1q. So again on switch 3 if we got to go up here I noticed trunk desirable
dot1q, dot1q whereas on switch2 trunk auto dot1q negotiated. So notice desirable dot
1q, dot1q trunk auto dot1q negotiated, one side was forced to use dot1q the other side negotiated
to use dot1q. Okay, so, in summary, left end side doesn't become a trunk because both sides are set
to auto right hand side does become a trunk because one side was set to dynamic desirable as we can
see over there and uses dot1q on this side show interface trunk the only trunk that was negotiated
is on gigabit 01. Okay how are you doing?
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