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At this point, we have a couple different modes of operations for our switch

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ports.

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And we're talking about switch ports, we're talking about a layer two

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interfaces.

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So we have, for example, access mode, which is going to operate without tags.

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So this is an untagged mode of operation.

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And if I'm going to not tag something, then I have to tell the switch which V

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LAN that's running in.

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And so I'd say, hey, you're running in VLAN 10.

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And then all traffic into and out of that interface is considered to be part of

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that VLAN.

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And then, of course, I do have the trunk mode of operation.
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And then, of course, I do have the trunk mode of operation.

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And this is going to use our VLAN tags.

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And ideally, in some way, we're either tagging the native VLAN, or we are not

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using a native VLAN or using a dummy VLAN as we talked about.

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So these are our primary two options.

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The access ports are intended to go to our end hosts.

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Because our end hosts, our PCs, our laptops and such, these are not usually

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tagging traffic.

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I just want to send my traffic.

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And I don't want to have to worry about what VLAN ID I'm on.
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And I don't want to have to worry about what VLAN ID I'm on.

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And so our hosts just send untagged traffic to the network and allow the

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network to figure out what VLAN it's a part of.

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Meanwhile, trunks are going to be used between switches.

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And at that point, I want to make sure that I have configured my interfaces for

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trunks so I can carry multiple VLANs.

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But here's the question.

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Do we want this to be a manual or static configuration?

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Because if so, then we need to make sure that we're performing lots of
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Because if so, then we need to make sure that we're performing lots of

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configuration.

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I have to get onto every single interface and statically map it as trunk or

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access.

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And I can do that simply by using that switch port trunk command again, this

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time with switch port mode.

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I guess I said trunk, switch port mode, and then I either say access or I say

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trunk.

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And that is how I do it, but I have to get onto every single port and make sure
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And that is how I do it, but I have to get onto every single port and make sure

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that it's all properly configured.

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And I will say that in the real world for the most part, this is what I see.

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We typically are getting onto our ports and it's not that hard to figure out

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which ports connect to other switches

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and make sure they're configured for trunks and everything else is access ports

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.

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And that's all fine.

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But we should know that we do have another option.

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We do have this concept of dynamic trunking.

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Dynamic trunking is going to allow us to avoid this static world where I have
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Dynamic trunking is going to allow us to avoid this static world where I have

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to know which ports are access and which ports are trunk

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and just allow the switches to figure it out.

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It's kind of a cool idea.

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So here's the idea.

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If I have switches like this and let's say I've got a bunch of ports downstream

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and maybe a port over here to the other switch.

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With dynamic trunking enabled, the switch on the left here, we'll call this

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switch one, figures out that these ports need to run in access mode.
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switch one, figures out that these ports need to run in access mode.

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And this port here in the middle needs to run as trunk.

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I'm using quotes mostly just to represent the fact that I didn't have to

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manually configure that.

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And switch 2 is going to figure out the same thing and say, hey, there's a

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switch on the other side here.

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We should run this as a trunk and these ports are going to have to run in

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access mode because I don't sense another switch out those interfaces.

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So it's a way for me to have to avoid getting on to every single switch and

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telling it exactly which ports are access and which ports are trunk,
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telling it exactly which ports are access and which ports are trunk,

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even though it does get a little bit away from the concept of intentionality

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that I brought up earlier.

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But it is an option nonetheless.

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So how exactly does dynamic trunking work?

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Well, it's going to rely on a protocol known as the dynamic trunking protocol

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or DDB.

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And the idea here is that these two switches are going to be sending DTP

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messages out all over the place saying, hey, is there another switch out there

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on the other side?
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on the other side?

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Now notice that there is no switch down here on these sides. So if we're

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sending DTP messages out and not getting anything back,

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then at that point we know that these must be access ports.

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Meanwhile, this connection in the middle, we are getting a DTP message back and

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we recognize that there's a switch on the other side.

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And as long as both sides agree, we can form this into a trunk.

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And so it's as simple as sending some messaging back and forth and seeing if

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there's a device on the other side that mutually wants to form that trunk.
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there's a device on the other side that mutually wants to form that trunk.

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So if I want to use DTP, I'm going to take a look at the switch port mode

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command, but I'm going to get another option.

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Instead of just access or trunk, I now get a dynamic option as well.

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And from here, I can specify two different options.

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I can specify desirable and I can specify auto, which honestly doesn't really

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tell me much about what's happening here.
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tell me much about what's happening here.

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I mean, auto means automatic, desirable makes it sound like it wants something,

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maybe wants to form a trunk and that's exactly what it is saying.

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But let's explore this in a little more detail.

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When we're talking about desirable and auto, we're really saying, do I want to

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initiate creating a trunk?

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So desirable means that I will initiate a trunk and auto simply means that I

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will allow a trunk.

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Notice that in either case, we will form trunks if we have this dynamic enabled
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Notice that in either case, we will form trunks if we have this dynamic enabled

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and there's something on the other side that's initiating.

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But here's where it gets interesting.

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So if I have a desirable on one side and it matches up to desirable on the

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other side, well, we both want a trunk and so we're going to form one.

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Meanwhile, if I have desirable and auto on two different sides or vice versa,

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either way, well, I've got a side that says I want the trunk.

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I've got another side that says I'll allow it and therefore we form trunks.
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I've got another side that says I'll allow it and therefore we form trunks.

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Lastly, if I have an auto and an auto, notice we'll both allow a trunk, but

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neither of us are going to initiate a trunk.

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This means that there will be no trunk formed.

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As a result, if I really just want to make sure I form trunks everywhere, I

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should probably just configure dynamic desirable on all of my interfaces.

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However, if there are interfaces where I don't want to be aggressive, I just

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kind of want to allow it, then that's where I would use auto.
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kind of want to allow it, then that's where I would use auto.

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Now, when we configure port channels and specifically LACP later, we're going

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to find that there's a very similar methodology as far as one side initiating

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the other side,

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maybe just allowing but not initiating. This is the same kind of concept, but

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we'll again, we'll see that when we talk about LACP coming up.

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Now, one important note in all of this is that when we first turn on a new

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Cisco switch, we can look at the interfaces and the interfaces have a default
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Cisco switch, we can look at the interfaces and the interfaces have a default

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configuration.

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What is that default configuration, you might ask? Well, believe it or not, it

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is actually using DTP.

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Even today, where a lot of organizations don't rely on DTP, we don't always

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realize that it's actually still running in the background.

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So the default is to run in this switch port, well, we'll just cut it down to

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say, we're running in auto.

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Switch port mode dynamic auto is what our default configurations are.
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Switch port mode dynamic auto is what our default configurations are.

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And so I can get on there and say, hey, I want to manually configure a trunk

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link, but believe it or not, it's still running DTP as overhead.

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And so if I want to disable DTP, I do have a command for that, and it is switch

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port no negotiate as all one word.

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And I know that when I've deployed network configurations, if we're not

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intentionally using DTP, which was pretty much never.
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intentionally using DTP, which was pretty much never.

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In my, at least in my neck of the woods, I would always include this as part of

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my default configuration because, again, intentionality matters, and I don't

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want to protocol running if I'm not making use of it.

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So if we're not going to use DTP, we should probably consider disabling it. If

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we are going to use it, recognize that the default is in auto and we are still

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going to have to manually configure all those ports to be running in dynamic

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desirable.

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