So let's carry on with our discussion of Spanning Tree basics. We just talked about how the Root Bridge is generally the oldest device on the network and you can fix that by using the priority because obviously you don't really want to modify the MAC Address, although I suppose it's technically possible. But the recommended way of modifying which Bridge actually gets elected the Root Bridge is by modifying the priority. So let's move on and let's talk about Port States and Spanning Tree. Now again this is just the rote learning section of the course here, you'll actually see these Port States when we put this in a lab and it'll make a little more sense or at least it did to me. Because I, I read this preparing for the switch exam and preparing to record this course and while I logically understood the descriptions, it really didn't make much sense until I saw it in the lab. The first is Listening. Now the Listening is what the initial state is when you first plug a device up or when a Switch Port first comes online. It just sits there and it listens for BPDUs to be received on the port and it waits for 15 seconds. Now if a switch receives any BPDUs in that 15 seconds then the switch knows, well hey there's another switch on the other end of this link, I'm going to send him a BPDU and we're going to negotiate which one of us is the designated port for this segment. And we'll get into the Designated Port is here at the bottom part of this slide. If the switch does not receive any BPDUs on a given port within that 15 seconds, then it transfers to the Learning State. The Learning State is just that, it's the switch sitting there listening for what MAC Addresses might be off this particular port and it will sit there and learn for 15 seconds. And these times are the defaults, obviously you can modify them but most people don't. Most people actually don't run regular Spanning Tree, they run Rapid Spanning Tree but we'll get into that later. After it goes from the Listening to the Learning State, it will finally transition to the Forwarding State, normal operations, port is passing traffic. However, you can look at this and tell that it will not hit the Forwarding State for at least 30 seconds. So if you have a machine that's plugged into a Switch Port, you power that machine on and you will not actually be able to transfer any traffic from your machine out to the network for 30 seconds. At which point, your workstation has already booted up, it's tried to get a DHCP address, since it can't transmit any traffic, it just says, well I can't contact a DHCP Server, I'm going to use my automatic IP address assignment. So you'll get an IP address of 169.254.something.something and you won't be able to access any resources on your IP network because you don't have a valid IP address for that range. And that's because unless you modify the way Spanning Tree works, it will wait for 30 seconds and not let any traffic in and out. Now Cisco has come up with an enhancement to Spanning Tree called PortFast and if you go into the Switch Configuration and you go into the Interface Configuration for an Edge Port and you put the Spanning Tree PortFast Command in there. Then it will first off bark at you, saying hey you don't really need to do this unless you absolutely sure it only connects to workstations, but once you enable PortFast, then it skips the Listening and the Learning State and goes immediately to Forwarding, immediately upon the Switch Port coming up. So obviously you can see how that could produce loops in the network, if you enable PortFast on the wrong Switch Ports. The last Port State we'll talk about is Blocking. Blocking in Spanning Tree means, hey if I pass traffic on this port, it's going to cause a loop in the network and so I'm just not going to forward any traffic across this. Now we'll see this when we put this in a lab but on an Ethernet Segment where you have two switches that are on that same segment. One Switch Port on that entire segment will be the designated port where all of the traffic for the network goes out and all the rest of the Switch Ports on that segment will be in a Blocking State. Because obviously, if the switch accepted or transmitted traffic on those ports, you would have a loop in the network. Now we've talked about the Port Roles a little bit when we talked about the Port States. Let's look at the actual definition of the Port Roles. The Root Port is the port that points at the Root Bridge, it's the lowest cost port determined by the Path Cost. I've got a table that shows you what some of the more common Path Costs are on the next slide. The other Port Role in Spanning Tree is the Designated Port. It's the port on a segment that forwards traffic for that segment, it's determined by the lowest Bridge ID on the segment. So if you've got five switches on a particular Ethernet Segment, they all send BPDUs back and forth, the one that has the lowest Bridge ID becomes the port that all the traffic for that segment comes through. Talked about Port Cost, there are some of the cost values for some of the more common port speeds. Obviously the higher the cost the less likely that a Switch Port will be used. For example, if you have a 100 megabit port and a 10 megabit port, both connected to the same switch, they're both connected to the Root Switch for example. Then obviously the traffic will prefer the 100 megabit port because it has a lower cost value, just like it would prefer the 1 gigabit port because it has a lower cost value. And of course it only makes sense, if you have a choice between 10 megabit or 100 megabit, well you're going to pick 100 megabit for, for sending traffic across. We'll conclude 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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