In this video we're going to put a nice little bow on our discussion of VLANs and VTP. We'll talk about cleaning up our VLAN Trunks and I know you might be thinking, well I set up these trunks, I just auto negotiated, they passed the traffic I need to pass, why would I need to worry about cleaning them up? They, they just kind of work. Well consider this, in the background I have gone into these three switches and I've defined VLAN 20 on Access 2, VLAN 10 on Access 1 and VLAN 15 on Core 1. And when I say I've defined them, I've put ports in those VLANs, so those three VLANs have ports spread across these three switches but each switch only has the ports for it's VLAN. Now let's say a packet enters from the Core Router, it's a broadcast packet, it enters Core 2 because Core 2 is in all three VLANs because it has to trunk the traffic over to the rest of these switches and it enters marked as VLAN 15. Well obviously it will flood this broadcast over to Core 1 because that's where VLAN 15 is. It will also flood the packet to Access 1 and Access 2 because VLAN 15 is defined on these two switches as well, even though there's no ports in VLAN 15 on either of these switches, the rules say that the broadcast traffic's going to be trunked over there because you haven't told it otherwise. Now you multiply that by 50 or 60 or 80 VLANs and 3 or 4 or 500 workstations and you can see how broadcast traffic going to VLANs that don't have ports on them on these switches can add up to a pretty significant amount of traffic. So how do we prevent this from happening, well there's two ways to do it. And one if you using VTP, you can turn on VTP Pruning and the second way, you have to think about where the trunks are and you have to define it manually. So the first thing we'll do, is we'll test out VTP Pruning, so we'll go back into our lab topology here and we'll go to our VTP Server, in this case, it's Core 2. I will go into Config Mode, Config T, we will do VTP Pruning and Pruning is switched on. So now if we go over to Access 1 for example and we do Show VTP Status, we see that Pruning is enabled. So now that we've given that a few seconds to settle down, if we do Show Interface Trunk, no we don't see anything down on the Access Switch because it's not actually doing any pruning. But let's look up at here at Core 2 for example and we'll do Show Interface Trunk on this guy. You'll notice that now unlike Access 1 where we've got all of these VLANs and the Spanning Tree Forwarding State and not pruned on Core 2, all that VLAN 15 is being pruned out on Fast Ethernet 01. And the reason why all of our VLANs are being pruned out is, even though there are ports defined on the Access 1 and Access 2 Switch for the other VLANs, those ports aren't up. For example, let's go over to Access 2 and do Show Span to see where our PC is attached. So Fast Ethernet 025, if I go into Config T Interface Fast Ethernet 0 slash 25 and let's change the Access VLAN on this guy to 20. So now if we do Show VLAN, we've got Port 25 and VLAN 20. We do Show Interface Fast Ethernet 025 to make sure it's actually up, so it's up, up. So now if we go back to Core 2 we do Show Interface Trunk, we have to wait a little bit for VTP to catch up with us, you'll notice that now it's not pruning out VLAN 20 because it says on your Access 2 Switch, you have a port that's in VLAN 20 and it's actually up and up. So as you can see VTP Pruning would just kind of take care of it for you, it realizes what VLANs are on what ports and on what switches and just doesn't trunk that traffic across those Trunk Ports if there's no switches on the other end to receive it or any devices on the other end, to, to be more accurate. Now the other way to do this is just by doing it manually and to do that, you'll actually modify the port that is trunking to the destination switch. So let's see on Fast Ethernet 01, that should go to Access 1, so let's look at Show CDP Neighbor to make sure and Fast Ethernet 01 goes to Access 2 actually. Yes because I'm on the Core 2 Switch, so if we go into Config T and do Interface Fast Ethernet 0 slash 1 and we are going to manually prune out VLAN 10 from this Trunk Port. And the command we use is Switch Port Trunk, we'll use the Allowed Keyword, because we're setting Allowed VLAN Characteristics when it's in Trunking Mode. So we'll do Allowed VLAN and now you can tell it, you know, just a list of all the allowed VLANs. You can say all VLANs, all VLANs except the following, you can add or remove VLANs. So that way, if you have a, a list of VLANs that's 40 VLANs long, you can say, just add 27 to that list or remove 44. In this case, we're going to do all of them except VLAN 10 which will allow 1, 15 and 20 to be trunked out of this particular Switch Port. Once you do that, if we do Show Interface Trunk you'll see that VLANs allowed in an Active Management Domain, Fast Ethernet 01, VLAN 10 is not allowed to trunk out that port. And like I said, this requires a lot more work because you have to look at your diagram and figure out well, VLAN 10 goes down this way, so I'm going to block it on Port Fast Ethernet 01 as well as 023 because it's not over here on this Core 1 Switch. In, in reality it would actually be on both of your Core Switches but you get the idea. And that concludes our discussion of cleaning up VLANs.
|Course:||Implementing Cisco IP Switched Networks (642-813 SWITCH)|
|Duration:||8.5 hrs / 102 lessons|
|Captions:||Available on CD and Online University|
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