Welcome into the discussion of Spanning Tree basics. In this video we're going to take a few minutes and review how Spanning Tree works and go a little more in depth than we did in the CCNA material and we'll follow this up with a discussion of enhancements to the Spanning Tree Protocol, namely the Rapid Spanning Tree Protocol. So the Spanning Tree Protocol prevents loops in a switched network. Now obviously any well designed network will have redundant links throughout the network so that if any one piece of equipment fails or any one piece of critical equipment fails to be more accurate. Then you'll have a redundant connection to another piece of core equipment and you only have a small portion of the network fail, if you have any network failures at all. Obviously all these links can't work all the time and that's where Spanning Tree comes in. Now the Spanning Tree Protocol is the 802.1D Protocol, it is an IEEE Standard and every switch out there that's been made in the last 20 years supports it. The reason why is that the IEEE 802.1D Protocol was originally drafted in 1990. What were you doing in 1990? I was in Tenth Grade which just exposed my age for all the world to see. So what are the specific problems that are actually fixed by the Spanning Tree Protocol? The first is Broadcast Storms and that is the most obvious issue or the most obvious indicator that you have a loop in your network. You plug up a cable and suddenly all of the activity lights on all of your switches just come on and stay on solid and that happens when the same frame is forwarded around the network over and over and over. You'll recall when we discussed the various Network Models that the first slide that I showed you was a small business that accidentally connected two of their Linksys or 3Com hubs or switches together and all of the activity lights on all of their switches just turned solid. Now this is the biggest problem that Spanning Tree solves. The other ones that are not as quite as obvious is MAC Table Instability. MAC Tables and Switches can be confused by network loops, think about when a packet loops around the network. You have a Switch Port that receives the traffic from one MAC Address and then you have that same MAC Address because it's been looped around and coming in from a different direction visible on another Switch Port. Well that just throws the MAC Table inside the switch into a tizzy because you've got MAC entries and Arp entries being overwritten and some of the smaller switches that don't have a whole lot of processing power or memory, you can just completely lock them up by having MAC Table Instability. And the last is Multiple Frame Transmission and that's essentially where the individual end station, the computer or the IP phone, the video conference system, receives the same packet two or three or five or ten or twenty times. If you have this problem on your workstation, you'll go into Task Manager if it's a Windows box for example. You'll go into Task Manager and look at the CPU utilization, you'll see that CPU utilization is pegged at 100 percent because that workstation is processing the same broadcasts and the same frames over and over and over and over and you're networking stack will just drive your Windows system nuts. So how does Spanning Tree work? Well Spanning Tree, if you look at it, as a Switching Protocol, then you'll realize that it works pretty much like a Routing Protocol. There's lots of similarities and if you've taken the CCNA course or if you've done the route curriculum before you've gotten to the switch curriculum, if you think about Spanning Tree through the lens of a Routing Protocol, you'll see that there's quite a number of differences. Number one, Spanning Tree sends Bridge Protocol Data Units or BPDUs around the network and those are basically like little sonar packages or little switch updates that says, here is my path to my root. Here's my MAC Address or my Bridge ID, we'll see what the Bridge ID is here in just a bit and these things are sent out by default every 2 seconds. These switches use the BPDUs to determine the network topology, I'm getting BPDUs from five different switches, well four of these switches have a path to get to the root that costs 200, this one switch over here has a cost of 100. He's closer to the root, therefore I'm going to make this port pointing at that switch the Root Port. And we'll get into that when we discuss exactly how Spanning Tree chooses Root Ports and the path to the root and so on and so forth. Spanning Tree elects a Root Bridge based on the Bridge ID. What is the Bridge ID? The Bridge ID is an 8 byte field in the BPDU packets. Two bytes are the priority and six bytes are the system ID and the system ID is basically the MAC Address of the switch. The Root Bridge is the switch with the lowest Bridge ID. Now let's sit and think through this logically. Now MAC Addresses usually go in ascending order from a manufacturer. If Cisco's going to manufacture 500 switches, they'll start with MAC Address 1 and they'll go to MAC Address 500 in order. So therefore the older devices have a lower MAC Address, the priority is the same by default out of the box, it's 32,768 as the value and we'll see that when we look at the lab elsewhere in the course. So if we sit and think about this, then obviously the Root Bridge will be the oldest switch in the network. Not the switch that's been up the longest but the oldest piece of equipment on the network, that's generally not the switch you'd want to be the Root Bridge. We'll carry on with our discussion of Spanning Tree basics in the next video.
|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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