So we're carrying on with our verification and configuration of Spanning Tree. We've just realized that using the Default Configuration that this Access 2 Switch, the oldest, slowest switch in the network is the Root Bridge. Obviously this is a suboptimal configuration for several reasons. Number one, the traffic flow is obviously not the most efficient which we covered in the previous video. The second reason why this is not the most efficient, is that in normal circumstances, this oldest Access 2 Switch may not have as hefty of a back plane as one of these Core Switches. Now my Core Switches happen to be 3560s which are technically also Edge Switches just like these 2950s and 2960s that are down here and for a lab, obviously I'm not going to go out and spend the money to get Nexus 7000s or 6500 series Catalysts to put up here as the core. But in a production network these Core Switches would be big, hefty, expensive Layer 3 or above switches. The back plane on these things would be multiple gigabits being able to handle multiple thousands or millions of packets per second across the back plane. This Access Switch down here probably not so much, so basically what would happen is, as users came in and started using the network and as the day wore on and you started getting more traffic going across your network, the back plane on this Access 2 Switch would get overwhelmed. You would then run the CPU up high, you'd run the buffers out and you start experiencing just general lag and general slowness on the network. You know the network would function for the most part and it would be like, you know, something's just not right with my network. It, it seems to get slower when ever people get here and start using it for the day and you can't really track down what's going on. I mean the traffics getting from Point A to Point B, it's just taking longer than I think it should but I, I can't figure out why it's taking longer. Until you go in and look at Spanning Tree and you realize well jeez, this, you know, 12 year old switch is the root of my Spanning Tree down here. So how do we rectify this? Well obviously we know that the root of Spanning Tree is the switch with the lowest Bridge ID and the Bridge ID is based on the MAC Address and the priority. Now we can't change the MAC Address or we can't change it easily. So what we have to do is lower the priority of whatever switch we want to be the root of the Spanning Tree. Now I'm going to arbitrary pick Core 2 as the root of Spanning Tree because it does actually know about this Core Router. Obviously in an actual production network, this Core Router would have an Ethernet connection to both of the Core Switches or to be even more redundant, you'd have multiple core routers going into your WAN provider. Since this is a lab, I just opted for the single Ethernet connection to keep things simple, specially since we're not going to be routing anything off, of our network. We're only concerned with the LAN here because this is the switch exam after all. So let's go up here on Core 2 and let's examine what switches are forwarding and blocking and what not on him. So let's go back to our lab PC, we'll go back to Core 2, we'll do Show Spanning Tree on this guy. We'll see that Fast Ethernet 023 is an Alternate Role. The Alternate Role basically says this is a valid Switch Port, it's up, it's connected to another switch but I can't forward traffic out of it because it would create a loop in the network and you'll see the status is in Block. And if we go back to our diagram, I'll put all those back on there. We'll see that Fast Ethernet 023 is this link between Core 2 and Core 1. So obviously since it's not the root, it picked that interface as being the one that gets to get blocked. So let's fix that. So what we'll do is, we'll go into Config Mode and let's examine some of the Spanning Tree Commands available to us. So if we do Spanning Tree and hit Question Mark, we see we've got several options, most of which we'll cover when we talk about advanced switching, that's the Backbone Fast and the Loop Guard and all these other commands. What we're looking for is the VLAN. Now even though we've not set up VLANs, the switch comes with a default VLAN or a VLAN 1 set up. If you take a switch out of the box with no configuration whatsoever and hook it up, you'll have VLAN 1 set up on all your switches. That way, you can take five Cisco switches, take them out of the box, hook them all together, hook servers and workstations and everything up to them and they'll just work with no configuration. And so that way, if you have a small network or if you're in like a disaster recovery situation where you've got set up a server farm real quick and just get some people connected to it to set up a call center for example, you could just grab and go without having to spend minutes or hours configuring all these switches. So in this case, we're going to change this Core 2's priority for VLAN 1. So we're going to do Spanning Tree VLAN 1 and we could just go manually set the priority right here and say Priority and we could say 0 to 61,440, Bridge Priority and increments of 4096. Of course the Default Priority is 32,768 or pretty much right in the middle. However there's an easier way to do it and that is to simply say Spanning Tree VLAN 1 Root. And we can say that this is the primary root or the secondary root for the Spanning Tree and this allows you to set up a little bit of redundancy in your Spanning Tree Configuration as well. This way if the primary root goes down then you know, hey if it fails, I know which switch it's going to fail to, rather than, well just pick one, because everybody else is at the Default Priority. So we'll do Spanning Tree VLAN 1 Root Primary and hit Enter and we've just called Spanning Tree to re-converge. So I'll give it a few seconds to shake itself out and then we'll look at how Spanning Tree's configured now. Now that should be more than enough time. So let's go ahead and exit Config Mode and I'll write my Config just to be on the safe side. Now if we do Show Spanning Tree, we see that this Bridge is now the root and look what it did to the priority. It lowered it to 24,577, if we had not specified that Primary Command, it would have lowered it to somewhere around 287, I think is what it could lowers it to. You'll notice that all of it's ports are now forwarding, they're all designated for the segment. There all forwarding because this is the Root Switch and that's the grand prize for winning the Root Switch election is to have all of your ports forwarding. So we'll pause here for a second and pick this up in the next video.
|Course:||Implementing Cisco IP Switched Networks (642-813 SWITCH)|
|Duration:||8.5 hrs / 102 lessons|
Vista/XP/2000, OS X, Linux
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