CCIE Topic: 1.1e MST

hey everyone i'm charles judd and welcome to this video covering the 1.1e blueprint topic of multiple spanning tree or mst when it comes to spanning tree this is at the top of the food chain for layer 2 loop prevention so let's take a look at the theory and implementation of this protocol in the previous two videos under subsection 1.1e we've already looked at pvst plus and rapid pvst plus and we saw how these spanning tree modes map a single spanning tree instance to every vlan in the network so if we have 50 vlans we'll have 50 spanning tree instances now although our modern networking equipment is much better at handling the overhead that's caused by this it really is a waste of cpu and memory resources and it can also cause configuration or troubleshooting nightmares and that's exactly why multiple spanning tree or mst was developed originally cisco developed mistp which stands for multiple instances spanning tree protocol and our modern mst is an industry standard that was inspired from this msc is defined in the ieee 802.1s standard and again this was created to enhance performance primarily by reducing the overall number of spanning tree instances in our network so if we have 50 vlans we could assign 25 of those into one mst instance and the other 25 into a separate mst instance so that we would only have two total spanning tree instances where path calculations are taking place and that's opposed to having 50 separate instances as we would see with something like pvst we can group network switches together by a mechanism called the mst region and mst regions can be thought of really in a similar way as we think of autonomous systems in bgp for example so they're just a way that we can group our switches together for common administration purposes every switch in the network running mst must be configured with certain attributes and in order for switches to share a region those attributes must match first they would have to share the same alphanumeric mst configuration name this is used to identify the region and you can make that anything you like for really easy identification purposes they should share the same mst revision number as well again you can make this anything you like starting out i typically start with the number one because that just seems logical to me but the general idea is that you can change this revision number every time you make a configuration change now this doesn't have to be changed when you alter the configuration it won't be changed automatically but that's simply something that you can use as a change tracking mechanism if you choose to do that and they should have the same vlan mapping table so this table maps an mst instance to a specific set of vlans on the switch and we define those manually during our configuration which we'll look at in a moment if any of these attributes do not match then the switches are automatically considered to be members of different regions within any mst region we're always going to have one default spanning tree instance called instance zero and that is our internal spanning tree or our ist this ist creates a loop free path within a single region so if we have two separate regions that means we're going to have an ist instance for each of those regions by default when we enable mst all of our vlans will be placed into instance 0. this is the same concept that we see with our switch ports where we see those start out in vlan 1 by default and just like those vlans it's a best practice to explicitly define separate instances for vlan grouping outside of instance 0 and that's because we have the potential of losing some benefits of mst if we leave everything in the default instance when we do create new instances into which we assign our vlans these are called mstis multiple spanning tree instances we can number these 1 through 15. so cisco supports 16 instances one of course being instance zero which is our ist and the other 15 which we can define as mstis a couple more terms to know first the common spanning tree or cst this interconnects all instances of spanning tree in the network so this bridges mst regions with instances of 802.1d and 802.1 w spanning tree there will only be a single cst instance in cases where this is needed so this is essentially a mechanism used to present an mst region to a switch in a different region in a very simple manner and we have the common internal spanning tree or c-i-s-t this is a collection of the ists in each of the mst regions two switches inside the mst region this is exactly the same as the ist two switches outside of the mst region this appears exactly the same as the cst so that's lots of theory there's lots of acronyms i've thrown around there so take some time to rewatch that if you need to and to fully digest everything that's happening and let's jump into a lab and see how we can configure this our topology is going to be fairly simple we have three switches with redundant trunk connections between those and we're going to have vlans 10 20 30 and 40. in this lab i'm going to configure mst on all three of these switches and i'm going to create two instances of spanning tree i'm going to add vlans 10 and vlan 20 to instance number one and lands 30 and 40 to instance number two fairly simple so let's do that okay here on switch one let's run a quick show vlan brief command and here you can see that vlans 10 20 30 and 40. those already exist and i've configured these on my other two switches as well so next let's enable mst on each of our switches first let's say show span to take a look at our current spanning tree configuration and we're going to of course see instances for the default native vlan of vlan 1 we're going to see an instance for vlan 10 vlan 20 vlan 30 and vlan 40. we see all of those separate instances of course because we're running pvst mode currently so let's consolidate those with mst if we go under global configuration mode and we say spanning hyphen tree mode and we look at our contextual help options we can see the options we've previously examined and this time we want to say mst so let's break out of there and here let's say show spanning hyphen tree mst configuration and notice we can see this instance doesn't have a name that's because it is our default instance of instance 0 which we can see here and as we mentioned earlier all of our vlans are by default mapped into this default instance instance 0. we can also see that this is the only instance of mst configured so let's jump on switch 2 and let's enable this here as well under global configuration mode spanning hyphen tree mode mst we'll do the same on switch three spanning hyphen tree mode mst so that's good so now we have mst configured on all of our switches and what we want to do next is to create some different instances and assign our vlans into those let's go back to switch one let's go back under global configuration mode and we need to go under mst configuration mode next by saying spanning hyphen tree msc configuration you'll notice our prompt has changed to mst configuration mode letting us know that and remember those attributes that we need to assign which are the configuration name the revision number and the vlan mapping we need to create each of those so let's do that we can say name and i'm just going to call this configuration mst for simplicity we can set the revision number by using the revision keyword followed by a number zero through sixty five thousand five hundred and thirty five always just start with one so i'll do that and now we can map vlans to our instances so let's say instance if we look at contextual help we can give this an id of zero through four thousand ninety four zero is already the default so i'm just gonna call this instance 1 and we can assign now vlans so let's say vlan followed by the numbers that we want to assign to this instance which of course for me are vlans 10 and 20. then we can hit enter very simple to do let's create instance two now and let's assign our other two vlans vlan 30 and 40 to that instance let's hit enter we'll break out we'll say show span mst configuration and now you can see we have three instances we have of course our instance zero where all of our unassigned vlans are mapped to and we have instance one for vlan 10 and 20 and instance 2 for vlans 30 and 40. so that all looks good let's now say show run pipe to begin spanning hyphen 3 mst this is just going to give me a look at the configuration so we can see that here this is just going to allow me to save a little bit of time by copying this i'm going to copy that i'm going to paste that into switches 2 and 3 just to save some time with configuration so there we go got those pasted in place that looks good let's go back to switch one now let's break out of the show command and let's say show span so now you'll notice we're told spanning tree is enabled with the mstp protocol we see instance zero mst0 containing of course the vlans that we haven't assigned to anything and we'll see instance 1 mst1 and finally instance 2 mst2 so rather than having a separate instance for each of our vlans here we only have three instances now our default we have instance one and instance two so we've successfully consolidated spanning tree in our network just a couple more things to look at let's say show span mst 0 and this is going to show us information specifically about an instance that we indicate in this case by choosing instance 0 we're looking at the ist the internal spanning tree and that's why we actually see this message telling us that this switch is the route for the cist the common internal spanning tree if we arrow up and do the same for instance one you'll see that this is also the root switch for instance one and if we look at instance two we're going to see the exact same thing this switch is the root for instance two as well and of course that's because we have the lowest mac address in the network now we can influence this by changing the priority specifically for an mst instance let's jump over to switch 2 spanning hyphen 3 mst for instance 2 if we look at contextual help you'll see that we have a couple of options here we can manually assign a priority value using intervals of 4096 or we could simply say root and we could indicate this as a primary or a secondary route just like we looked at with pbst plus this would allow the switch to dynamically assign a priority value now the consideration with using this dynamic assignment is that later if we introduce a new switch into the network that has a lower priority value than this root switch that is going to become the root bridge and this dynamic assignment is not going to take any further actions it's not going to continue to calculate that it only calculates that at the time that you enter the command so if we want to be absolutely sure that this won't change then what we want to do is to statically assign a priority and if we look at this if we say priority and look at contextual help again we have to do that in increments of 4096 and we can even use zero as our priority to make sure that this is always going to be the root bridge let's now say show span mst2 and you'll notice that this switch is now the route for mst 2 just as we would expect to see so that completes a look at mst multiple spanning tree protocol i hope you found this content useful and i want to thank you sincerely for watching

Channel: Charles Judd

Views: 1,936

Keywords: cisco, ccie, cisco enarsi, ccie enterprise infrastructure, cisco enarsi 300-410, cisco encor 350-401, ccie lab, my ccie journey, ccie training, ccie blueprint, spanning tree protocol, stp, spanning tree, PVST+, per vlan spanning tree plus, rapid pvst+, 1.1e spanning tree protocol, multiple spanning tree, mst, rapid per-vlan spanning tree plus, per-vlan spanning tree, root bridge, spanning tree priority, spanning-tree

Id: gNspf8nkz6c

Length: 13min 50sec (830 seconds)

Published: Wed Aug 12 2020