Implementing Cisco IP Switched Networks (642-813 SWITCH) Tutorials
Switch and VLAN Basics / VLAN Trunk Negotiation
Subtitles of the Movie (Implementing Cisco IP Switched Networks (642-813 SWITCH) / Switch and VLAN Basics / VLAN Trunk Negotiation)
In this video we are going to discuss Trunk Negotiation. Now this is not to be confused with VTP or the VLAN Trunking Protocol although there is an aspect of VTP that does come into play when you talk about Dynamic Trunk Negotiation. In my opinion VTP was kind of misnamed, it's, it's kind of misleading because you're not really negotiating VLAN Trunks, you're really negotiating VLAN Configurations but we'll get into that when we discuss VTP elsewhere in the course. Dynamic Trunk Configuration on Cisco switches uses the Dynamic Trunking Protocol or DTP. Now DTP is Cisco proprietary protocol and it is used to determine whether or not a Switch Port will be a Trunk Port or an Access Port depending on the settings on both ends of the switches. So let's just talk about the Trunk Negotiation Settings and we'll just plow right through them here. The first is Auto and Auto means, I'll become a Trunk Port but only if you want to become a Trunk Port. I kind of get a mental image of the goofy gophers from the Warner Brothers cartoon, oh no, no after you, oh I insist you must go first, no no indubitably you have to go first. No, no after you Alphonse, anyway, if you have two switches that are set to Auto on both ends, they'll never form a trunk because their waiting for the other end to initiate trunking. Now that's something to remember if you're ever troubleshooting Dynamic Trunk Negotiation, a lot of people will say, oh well it's set to Auto, it should just auto negotiate a trunk. Well Auto doesn't really do what it says it will do or doesn't do what you think it would do and so you have to go set it to something else, namely Desirable or Dynamic Desirable. Now this is the Default Setting from most modern switches and you'll see that in the configuration when we actually set up VLANs. Desirable or Dynamic Desirable will auto negotiate as an Access Port if I don't see any DTP packets coming from the other end. Otherwise I'll negotiate as a trunk. Now Desirable or Dynamic Desirable needs to be disabled on Access Ports, this gets back to the security of the network as well as the security of the network infrastructure. If you have someone who plugs up a Cisco switch in a cube someplace and they auto negotiate a Trunk Port, then in theory they could eavesdrop on any of the VLANs that happen to cross that switch. That's not really something you want to support for any number of reasons really. The next setting is On, that's I'm a trunk, whether you like it or not. You can send me non trunk data, I'll just ignore it or more accurately I'll put in the Default VLAN. This enabled using Switch Port Mode Trunk on the Interface Configuration and we'll see that when we set up trunks in our lab. The last setting and there is one more setting but the last setting that deals with DTP is off. I'm an Access Port, whether you like it or not, you can send me tagged data, I'll just drop it. You do this by saying Switch Port Mode Access in the Interface Configuration. The last actual setting for Trunk Negotiation is not really a setting in my opinion, it's Non Negotiate and that says don't send DTP packets and ignore any that you receive. This is how you disable DTP when you're talking non Cisco equipment. Now we mentioned earlier that there was one setting in VTP that came into play when you're talking about Dynamic Trunk Configuration and that is this error message here. Unable to perform Trunk Negotiation on this port because of VTP Domain Mismatch. Now again we'll talk about VTP Domains when we discuss VTP. Basically, you'll get this message even you force the Switch Port Mode to trunk because you're still sending out those DTP packets, part of the DTP packet is the VTP Domain. The upshot of all this is you can't set up a trunk between two switches in different VTP Domains with DTP enabled even if you force them both to be Switch Port Mode Trunk, you're still trying to negotiate DTP between the two ports and so you're still seeing those packets. Basically you need to disable DTP with the Non Negotiate Setting if the two switches you're trying to set up a trunk between are in different VTP Domains. And again, as I'm wont to do, we'll actually force this error in the lab so you can actually see it in production and you can see the two ways of fixing it. Either fix VTP or disable DTP, most people disable DTP because again, you want to know where your trunks are configured, you don't want them just to set up all willy nilly all around the network. And that concludes our discussion of VLAN Trunk Negotiation.
Tutorial Information
| Course: | Implementing Cisco IP Switched Networks (642-813 SWITCH) |
| Author: | Greg Dickinson |
| SKU: | 34304 |
| ISBN: | 978-1-61866-041-1 |
| Release Date: | 2012-04-20 |
| Duration: | 8.5 hrs / 102 lessons |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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