16. Rapid Spanning Tree Protocol 802 1W

hello and welcome to this tutorial we're going to talk about the rapid spanning tree protocol which is also called 802 dot 1w now this is an evolution of the 802 dot 1d spanning tree or the standard spanning tree that we've talked about so far so it's not a complete overall overhaul it's just it's just an evolution and what we're going to do is talk about some of the new terms that rapid spanning tree introduces and some of the changes in how rapid spanning tree functions and we'll compare them back to the standard spanning tree so that we can fully grasp the differences and the improvements now why did rapid spanning tree come along well even though we improved 802 dot 1d with features like port fast and uplink fast and even backbone fast there was still a lot of room for improvement because the demands on the network to converge faster and faster just keeps growing and growing so quite simply rapid spanning tree reduces the amount of time needed to converge and that's really the big improvement in this version of spanning tree and just keep in mind we're gonna see how rapid spanning tree does this exactly and it's not just a matter of changing the timer configs I know we mentioned in one of these standard spanning tree tutorials that you can adjust the max age timer and whatnot well there's a lot more to it than just that now standard spanning tree had a convergence time of about 50 seconds total with rapid spanning tree we usually get that down to below 10 seconds and usually it can be as good as 1 or 2 seconds so it's a drastic improvement let's begin by taking a look at what is similar with rapid spanning tree to the original spanning tree we've talked about so far first we still use the concept of the root bridge and we use the same methods to elect the root bridge and rapid spanning tree as we did before we also still use the concept of the root port on non root switches and we still use designated ports as well also we still use BPD use the hello bpdu in order to enable all these switches to communicate and share information now the next couple topics we cover will all highlight differences between rapid spanning tree and standard spanning tree the first one is the timers now with rapid spanning tree the max age timer is now six seconds as opposed to the 20 seconds with the standard spanning tree so that's a pretty big improvement the second change in timers has to deal with the forward delay timers now we don't reduce them in fact we completely get rid of them so in rapid spanning tree there is no forward delay timer and if you remember that is 15 seconds and we usually came across at two times once with the learning state and once with the listening state so in total we just shaved off another 30 seconds of convergence time so combined you can see already we're making some drastic improvements to rapid spanning tree next let's take a look at the different port states of spanning tree in other words how a port behaves in standard spanning tree we had five forwarding learning listening blocking and disabled now in rapid spanning tree we only have three we keep the forwarding and the learning States however the listening blocking and disabled States a standard spanning tree they all get merged into a single state which is now called the discarding state now a port in the discarding state will not send or receive any frames and it will not learn any MAC addresses however it can receive BPD use so it's similar in behavior to the blocking state of standard spanning tree as a study tip keep in mind whether it's standard spanning tree or rapid spanning tree the only states that can send and receive frames are the forwarding States there are also differences in the port roles between standard and rapid spanning tree standard spanning tree has the route designated and blocked port roles rapid spanning tree also has the route and designated roles however it's taken the blocked role in standard and it split it up into two different types known as the alternate and the backup now the alternate in the backup are still blocking but they do it in a different way and in different scenarios let's put this in a diagram to make it clear let's start with the alternate port now this is the next best route port and alternate to the route port and it'll be used if the active port fails so if you remember the uplink fast feature where we identify the next best port in case we need it well it's very similar to that that functionality has been baked into rapid spanning tree so here switch C and B are connected and the designated port is on switch C so switch B puts that into a blocking State however the port role is now known as the alternate port so should the route port on switch B fail it will now go ahead and use the alternate port as its new route port okay so very similar to uplink fast now the backup port this is used when a switch has two connections to the same Ethernet segment which is oftentimes done with a hub so here we have a hub and switch C has two connections to it well one of those two connections will be desi will be the designated port and the other one will be the backup port and that'll be in a blocking state as well however should the designated port fail then the backup port can be used to keep connected to that Ethernet segment now something new about rapid spanning tree that didn't really exist with standard spanning tree is how a switch will look at the connection between it and another device so with rapid spanning tree it actually cares about what a switch is connected to so for instance if two switches are connected via a full duplex Ethernet connection rapid spanning tree calls at a point-to-point link if a switch is connected to something like a PC a user on your network that connection is called an edge connection now if a switch is connected to a shared network like a hub here it's often called a shared link and each type of these connections is treated a little bit differently in rapid spanning tree especially when convergence happens basically the point-to-point link and the edge connection benefit from rapid spanning tree whereas the shared connections the shared link they don't really benefit from all of the improvements in rapid spanning tree we don't cover the details of the different physical connections and convergence in this tutorial but the main thing to walk away from is the different types of connections that rapid spanning tree has now that we've covered some of the basics of rapid spanning tree let's take a look at what happens when a change occurs on the network now in a stable condition rapid spanning tree is similar to standard spanning tree except every switch now can generate a Hello BPD you remember in standard spanning tree only the root bridge would create one and then everyone else would relay that BP to you well here with rapid every switch can create a hello bpdu and send it to its neighbours and so when a change occurs on the network where as standard spanning tree would just wait for hello B bpdu to arrive from the root bridge in order to figure out what the new root port should be rapid spanning tree takes a much more proactive approach to solving the problem and rapid spanning tree will actually actively contact its neighbor switches and together they figure out what to do so let's take a look at an example let's say a new link is provisioned between switch a and the root bridge and let's assume that this is now the best path to the root bridge for switch a it has a lower path cost now this is where the proactive approach is really noticeable in rapid in a tree the first thing that will happen is switch a will immediately block on all non edge ports except the one to the root bridge and this is known as the synchronization process so these links here if they are designated ports would go ahead and they would be shut down then switch a in the root bridge they're gonna put these ports into a listening state and by using rapid spanning tree they start to negotiate between each other and quite quickly switch a and the root bridge are going to agree to both put these ports into a forwarding state because this is now the best path for switch a to reach the root bridge and keep in mind during this time switch a has prevented any loops from happening because all of its other ports are in a blocking State next now that switch a has its new root port it's going to go ahead and send out it's updated B PDUs because it has a new root path too it's connected non-root switches E and B now let's take a look at switch B switch B and switch a will now repeat the same process that a just went through with the root bridge so switch B realizing now that it has a better path and let's assume this to be true to the root bridge is going to put all of its non edge ports into a blocking State and then it enters that negotiation phase with switch a now because this is the new best path for switch B switches a and B agree to put both of their ports into the forwarding State and switch B now has a new root port to the root bridge and so this whole process continues and if we were to look at let's say switch B and E now well this time switch E is going to ignore the new B PDUs from switch B and the reason why is that let's say switch e already has a direct connection to the root bridge and let's assume that it's the best path for it to the root bridge so it will ignore those PDUs and maintain its report to switch d ok so at this point you know this would cycle through the entire network so which is e F and C would all do the same thing and that would mean that convergence is complete and under create circumstances this can take as little as one second to occur all right well you've made it to the end of this tutorial so congratulations because we covered a lot of material so to summarize what we did cover we now know that rapid spanning tree gives us a much better convergence time compared to standard spanning tree and along with rapid spanning tree comes some new port states and also some new port roles rapid spanning tree also takes a look at the physical connections between switches and end devices and we also took a look at the actual convergence process itself and saw how each switch can proactively negotiate with its surrounding switches when a change occurs on the network so we're no longer waiting for everybody to receive a bpdu from the root bridge and then react rather once a changes is received on the network every switch can go ahead and actively help to resolve it quickly ok and so that's it that is rapid spanning tree thanks for watching

Channel: System Engineer

Views: 34,658

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Id: lB9s8ZELTU0

Length: 12min 30sec (750 seconds)

Published: Tue Apr 25 2017