Subtitles of the Movie

Let's pretend that we are a bridge that has just been installed into a network along with several other new bridges. So nobody knows who is in charge yet, and we would like to be in charge just as well as anybody else. So what we are going to do is, we are going to put our bid out there. We are going to say, OK my priority is x, and my MAC address is x, and I would certainly love to be in charge. Now all of the other bridges are going to do the same thing, and since nobody knows who is going to win yet, no data traffic is going to be transferred, only bridge protocol data units. So the bridges are all just flexing their muscles and listening to each other; they are presenting their bids to see which one will eventually be the lowest. Now this is a multicast transmission, it happens every two seconds, and it is initially used to set up a hierarchy, but then it continues because it's constantly looking for changes. It contains the bridge ID information which is used to control the state of each of the bridges' ports. So let's take a closer look at what happened there. Each of the devices went through four states and then ended up at one state; in other words, each of the devices went through a blocking state, initially they were all blocking because nobody was sending any traffic. All the ports were closed for business, except for bridge protocol data units, then they were all listening; they were all trying to determine, hey, am I going to end up in charge of this thing or not? So they were listening to each others bids; then once somebody was put in charge, they all learned the MAC address tables that were out there in the network; they all learned from each other, they all learned from the data traffic. And then some ports ended up in a forwarding state; they could only end up in a forwarding state, if they were going to be in fact the fastest way back to the Root Bridge, or if they were on the Root Bridge. All of the designated ports on the Root Bridge, when it's all said and done, will end up in a forwarding state. All of the designated ports on other bridges will end up in a forwarding state, but all other ports will end up in a blocking state. That way, what we were doing, just like the picture before, is we are setting up one and only one way back to the root, and we're cutting out the possibility for those loops. Now, are those ports still physically connected? Absolutely, and that is the benefit of the Spanning Tree Protocol. You see, we could disconnect these, once we determine - OK, we can do it this way - we could disconnect all these cables, but the problem with that, is that what happens if something happens here? What happens if something happens with the main connection? Then we have no fault tolerance; we are leaving the fault tolerance in place. What happens now is every two seconds, the system will check to make sure that everything's cool. If it's not, then it looks again and it says all right, well maybe we need to do this whole thing over again, elect a new Root Bridge and set up some new hierarchy, because what's happening is we've got a problem with one of these ports. But as long as everything's fine, then it would continue to operate in the way that it was set in the first place. So understand that all ports go through the blocking stage, the listening stage where they are receiving only bridge protocol data units, the learning stage where they are populating the MAC address tables. This allows them to get off to a fast start once everything does start sending. When they are populating their MAC address tables, understand they are not sending data, they are just populating the MAC address tables. And then some will end up in the forwarding state, and some will end up in the blocking state, and that's exactly what we wanted. So remember that the Spanning Tree Protocol was developed by Cisco, and the main purpose of it is to prevent loops while still maintaining fault tolerance in a network that has multiple switches and/or multiple bridges. So we can connect computers together in many different ways, and when we do, they need to be able to communicate with each other in the same language. Another way of looking at a language is a protocol. In our next chapter, we'll discuss local area network protocols. That's next. . Another way of looking at a language is a protocol. In our next chapter, we'll discuss local area network protocols. That's next.