Where we're going to take a look at trunking, We're going to discuss the theory of trunking and sometimes in production networks. We really don't have to do a lot to form a trunk. Sometimes, If we plug in the right cable, Everything is going to happen automatically and we really don't have to think about it.
But we all know for our cisco exams, Especially the CCNA exam. We better go a little beyond that and really know. What's going on with a trunk and why we need to create one in the first place, Let me bring up a quick diagram from the website. Actually, And here are just a simple example: you can call this a trunk line or just call it a trunk. If you will - but here we have hosts in the same VLAN, But t are connected to different switches and of course this is really what it's going to be like in the real world as well.
So we've got to allow our switches to communicate and send frames to each other, And that's really what a trunk is all about. Now, We've got a couple of different protocols that we can use with. Today's cisco switch is in order to form trunks. One is ISL and the other one, The full name is I Triple E 802 dot1q, But you're going to see it probably referred to on your exam simply as dot1q and that's how people usually refer to it in the real world as well.
There are three main differences between the two and we better know these differences, Like the back of our hand. First off is l is a Cisco proprietary trunking protocol where dot1q is the industry standard and, As I know here, Those of you, Relatively new to Cisco testing, Should get used to the phrase of Cisco, Proprietary and industry standard here. Just to mention here, If you're working in a multi-vendor environment, Then ISL may not be a good choice and even though is l is cisco's own trunking protocol, Their proprietary trunking protocol, Some cisco switches only run dot1q, You don't even have the option to run. Isl is l also encapsulate the entire frame, And this really does increase the network overhead and we're all is interested in keeping unnecessary overhead down. Dot1Q only places a header on the frame and, In some circumstances one major circumstance actually that we'll talk about in a moment. Even that header doesn't get put on so really there's much less overhead with dot1q as compared to ISL, And this leads to the third major difference.
The way the protocols work with the native VLAN, The native VLAN, All it is - is the default VLAN and sometimes I'll talk to a student he'll say: well, We don't really use vlans, We just you know plug the thing plug the switch ports up and we're ready To go, Even if you don't think you're using VLANs, You are because you're using the native VLAN or the default VLAN, Which I'll show you just in just one moment on cisco switches. That native VLAN is VLAN 1. Now, With dot1q frames that are destined for the native VLAN, T don't even have that header added to them.
So that's even less overhead. When that remote switch receives a frame with no hetero. It says: , This must be for the native VLAN. That's what dot1q the problem with ISL is that it doesn't even understand what a native VLAN is.
It doesn't understand the concept of it, So every single frame going across that trunk is going to be fully encapsulated. It's going to have a header and a trailer put on it and that's a lot of overhead. As I mentioned, We always want to bring up the live equipment whenever we can. So that's what I'm going to do here and we're going to take a look at a couple of cisco switches.
I have two cisco switches here that I have connected via a couple of crossover, Cables and t're, Both 12 port switches, And whenever you sit down for the first time at a switch, Maybe you go out in the field you need to see which vlans are being Used I like to use the show VLAN brief command. You can use, Show VLAN. There certainly nothing wrong with that, But you do get a little more information than you're generally going to need and if you run show VLAN brief I'll show you the difference here.
Use my up arrow and just type in brief here, You'll notice that we get a lot less information. Frankly, This is really all we need and you'll notice that VLAN 1 is the default VLAN. It's even named default and scroll over a bit and show you the ports that are in that particular VLAN and right now you can see that ports fast ethernet, 0 1 through 9, Are in VLAN 1.
So that's our native VLAN we're seeing everything we expect to see, But I mentioned this is a 12 port switch. So where are those other ports? If you see or don't see, Port numbers here, T're, Probably trunking and I'll, Show you a great command just to see which ports are trunking and it simply show interface trunk, And this is the information that you get and you can see up there at the top. We have three interfaces that are trunking, 10, 11 and 12 t're in dynamic, Desirable mode, Which means t're actively attempting to form a trunk with the other side we're using dot1q encapsulation. The status is that t are all trunking and note that the native VLAN is VLAN.
1, So this is a great command to see which of your lines are actually trunking and on this particular switch model of 2950. We don't even have ISL as an option so note that, Even though I SLO Cisco proprietary, It defaulted to 80 to 1 q and again that mode is desirable. So that's a good thing to keep your eye on. Whenever you run show VLAN brief, People tend to say, Wait a minute.
You know missing supports here, What's going on well, The only thing that's really going on is those happen to be your trunk ports and that's a really good place to get started. As far as your trunking goes, You can see what the motives which ports are trunking. What the encapsulation is. That's the trunking encapsulation protocol, What the status is and the native VLAN that's all there is to it.
So that is your review on trunking and again switching theory. I want to give you just a word of advice here, Because that's the first thing you really get into in your CCNA studies, If you're studying the icnd2, But when you first look at it's really the first big topic you have during your CCNA studies and it Can seem a little overwhelming at first my advice to you just break your studies in the smaller, More manageable parts, And sooner than later, You're going to see those magic letters CCNA behind your name, And there really is no better feeling want to invite you out to Some webinars that I'm running some free, Frame-relay, Webinars and other topics to we're going to run etherchannel webinars we've got a lot of great free webinars coming up just come out to ww bryant advantage com, /, CCNA, Webinars, Htl and the registration forms are right there. Also. I invite you out to the website where we've got over 275 free tutorials articles, Practice exams all kinds of great stuff in Microsoft, Server 2008 certification material is on the way as well.
Thanks for taking a few minutes to read this article, I'm chris bryant, Ccie, Number 129, 33 and I'll see you at the website.
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