Implementing Cisco IP Switched Networks (642-813 SWITCH) Tutorials
Switch and VLAN Basics / Overview of VLANs
Subtitles of the Movie (Implementing Cisco IP Switched Networks (642-813 SWITCH) / Switch and VLAN Basics / Overview of VLANs)
So we're continuing on with our review and overview of switching technologies and we're just about to jump into the wonderful world of VLANs. Now VLANs stand for Virtual LAN, kind of catchy ain't it. Well I might have named it differently, but they didn't ask me. VLANs allow you to split a single switch into multiple Broadcast Domains or basically separate Layer 3 networks. Now this solves the problem we talked about last on the previous slide, where you had too many devices in a single Broadcast Domain. This allows you to separate certain classes of machines, let's say you want all of the servers on one VLAN and all of your workstations on another VLAN which is actually one of Cisco's design practices they want you to follow. You can do that by configuring VLANs in the switch, which you'll see me do here in the lab, here in just a little bit. As a little bit of a graphical demonstration of how VLANs work and what they are, let's say you have six computers here and you've got three VLAN's. You've got the red computers here, the red VLAN and the green VLAN and this blue VLAN. Well obviously if this computer here sends a packet out, the traffic will only go out to the other computers on the red VLAN, in this case, it goes to this computer over here and this other computer over here on this same switch. This nice big purple link between the two is a Trunk Port and it allows you to basically have this one port on each of these switches in all three of these VLANs. So any traffic that comes in from blue for example, obviously there's no other blue ports on this switch, so it just gets sent across this trunk and talks to this other blue computer that's down here. Now that might raise the question, well what if you have to go between VLANs? Obviously VLANs separate them out into Layer 3 Networks, well you have to have a Routing Engine to switch packets between VLANs. Just as you would have to if it were any other type of Layer 3 Network, it just so happens that these two Layer 3 Networks happen to reside on the same switch or the same set of switches. But that Layer 3 Network could just as easily be the Internet or a WAN Link or any other type of Layer 3 Network. Obviously Layer 3 doesn't know, Layer 3 doesn't care so long as you get the packets to it and it knows what to do with them. Now traditionally the way this used to work is what was called a Router on a Stick and I actually searched Google Images to see if there was actually a picture of someone who took a router and taped it to a two by four and there wasn't one there. This is also called a One Armed Router and you'll see why here in just a second when I show you the diagram. So for example, let's say we have a switch here that's got three VLANs on it, VLANs 10, 12 and 15. We want to get traffic from this VLAN 10 to VLAN 15, well how do we do it? Well obviously these are separate Layer 3 Networks, if you send a packet from this VLAN 10 device, destined for VLAN 15, it'll get nowhere unless you have a Default Gateway or it'll get to your Default Gateway and not know where to go. However if you put a Router on a Stick on, you have a Trunk Port going over to this router and this router has VLAN Interfaces to find on it for all three of these VLANs and it has three IP addresses, one for each of these VLANs over here. So if you want to get traffic from 10 to 15, the traffic goes over to the switch, out this Switch Port to this router, the router then turns right back around and dumps it right back onto the same switch on VLAN 15. Now this works well enough and in a lot of networks, it's actually still in use today, just because of network inertia or for other political or technological reasons. Now this also has it's technical limitations, as you can imagine, you're limited by the bandwidth of this link right here. Even if this is a 1 gigabit link, you can very easily over subscribe this link by, if you have a lot of traffic going between all three of these VLANs on the switch, because it all has to go across this one link to get out to this router. And in some cases you also have memory and CPU and other issues over here on this router. Luckily Route Functions have been moved into the switch on modern switches for increased speed. As the technology has progressed, you've been able to take a router and a switch, merge them together into a routed switch, like that, you like my cool little graphic there? And so, if you want to move traffic from VLAN 10 to VLAN 15, it goes from VLAN 10 into this switch, into one VLAN Interface, Virtual Interface on this switch. The switch then routes over it's back plane, you know, it multiples of gigabits or 10 gigabits per second, out the Virtual Interface for VLAN 15, all of the routing functions take place right here inside the switch. There's no need to send it out to some external device, just to get the packets from one port to another. We'll set up both of these in our VLAN Lab when we set that up, the functions are pretty much the same, the end result is obviously the same but it's just two different ways of getting from point 10 to point 15, I guess. And that's it, that concludes our overview and review of switching in VLANs.
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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