CCIE Topic: 1.1e PVST+

hey everyone i'm charles judd and welcome back to the video beginning a look at subsection 1.1e for spanning tree protocol there's a lot of in-depth information covered under this item and also we're moving into some more advanced topics as we continue to go down the blueprint so i do expect these videos to start stretching out into multiple parts per blueprint section so this video is going to specifically take a look at per vlan spanning tree plus or pvst plus pvst plus or per vlan spanning tree plus is cisco's modern implementation of spanning tree traditional spanning tree creates only a single spanning tree instance for a network regardless of the number of vlans that are in place you'll recall that with traditional spanning tree there can only be one root bridge so this means that traffic for all of those various vlans would use the exact same path the original standard was pvst by the way which was used with cisco's proprietary isl trunking protocol and pvst plus was later developed to support 802.1q which is the industry standard for trunking that we've previously examined and basically what pvst plus allows us is to create multiple instances of spanning tree in our network and in fact we can create one spanning tree instance for every single vlan this means that we have the possibility of creating a different root bridge for every vlan in our network rather than having a single root bridge and each of those root bridges would handle all of the spanning tree path calculations for the vlan to which it is dedicated so if we had vlan 10 and vlan 20 as we're going to have here in this case each of those can have their own root bridge dedicated to those path calculations we can also create secondary root bridges for vlans as well so that we can build in redundancy to our network let's look at the topology that i'm working with here you can see that i have six switches and those are all interconnected with trunk links so we have some redundancy built in there as well and within this network we have two vlans we have vlan 10 and vlan 20. we have two host pcs connected to each of our access layer switches so we have a total of four end devices two of those in each of our vlans pc1 and pc3 those are in vlan 10 and pc2 and pc4 are in vlan 20. what we want to do here is to configure switch 1 as the root bridge for vlan 10 and we want to configure switch 2 as the root bridge for vlan 20. additionally we're going to configure a secondary root bridge as well we want switch 2 to be the secondary root bridge for vlan 10 and we want switch 1 to be the secondary root bridge for vlan 20. so basically if something happens to either one of these switches the other switch will be able to carry on as the root bridge for both of those vlans so let's see what we currently have in place we're on switch one and let's say show interfaces trunk and you can see that gig zero slash zero and zero slash one those are both trunk ports we can see the vlans allowed on those we're allowing both vlan 10 and vlan 20 as well as our default native vlan of vlan 1. let's also say show spanning hyphen tree and let's say summary and notice here that we are already in pvst mode it says the switch is in pvst mode so for newer cisco ios this is actually the default spanning tree mode and that's why we see that already in place if we go under global configuration mode here and say spanning hyphen tree mode and look at contextual help you can see our other options which we'll explore at a later time we have mst and rapid pvst but again by default our method is pvst so let's move on and look at that a little closer if we say show spanning hyphen tree you're going to notice that we can see our different spanning tree instances and we do have an instance for each of our vlans we have vlan 1 if we scroll to the top we have an instance for vlan 1 here and you'll see the root id ending in c 0 4 d that is not our local root bridge so switch one is not the root bridge for vlan one and you'll notice we have an instance for vlan 10 as well with the same root bridge id and vlan 20 with the same root bridge id again we can see this root bridge ends in c04d this particular local switch address is 927a so this is not the root bridge it looks as though gig zero slash zero is in the forwarding state it's listed as a root port and that means that this interface is closest to the root bridge we can see from our topology that this is interconnected to switch 3 so let's jump over to switch 3 and take a look there on switch three let's say show span and again we're gonna see our multiple instances of spanning tree running we're gonna see that the root bridge id again c04d the local bridge id is d316 so again not the root bridge but we can see that we have a root port designated as gig zero slash one from our topology we can see that is connected to switch five so it's pretty safe to assume that the root bridge currently is switch five but let's jump over there just to verify that we'll say show span and we are explicitly told this bridge is the route for vlan 20. this bridge is the route for vlan 10. and if we scroll up finally this bridge is the route for vlan 1 as well so it looks like switch 5 is our root bridge as it stands so again going back to our topology and what we actually want to configure we want switch 1 to be the root bridge for vlan 10 and we want it to be the secondary root bridge for vlan 20. so let's go back to switch one and let's look at how we can configure that let's go under global configuration mode and let's say spanning hyphen tree and if we look at contextual help we have several options here but the option we want is at the bottom we want to say vlan this is going to allow us to configure those per vlan spanning tree options and we want to follow that with the vlan number i'm going to start with vlan 10 we'll look at contextual help again a few things we can do here we can configure the max age interval we can adjust the hello interval we can set the priority but in this case i want to say root because i want to set this as a root bridge for vlan 10. if we look at contextual help once more we can set that as our primary or our secondary route and of course we want this to be the primary route for vlan 10 so i'll use the primary keyword and i'll hit enter now we can do essentially the same thing to configure a secondary route bridge for vlan 20. so we'll say spanning hyphen tree this time we want to say vlan 20 and for the root we want to set it as the secondary root bridge for vlan 20 and then we can hit enter let's go to switch 2 now and we want to do something fairly similar global configuration mode and we'll say spanning hyphen tree vlan 10 root we want that to be the secondary root bridge for vlan 10 spanning hyphen tree vlan 20 and we want it to be the primary route bridge for vlan 20. so that looks good all that's in place let's go back to switch one and clear off just a little bit of room and let's say show spanning hyphen tree and we can see again our multiple instances let's scroll up just a bit and you'll notice for vlan 10 now on switch one we're told this bridge is the root the root id ending in 927 a matches the local bridge id 927a this is in fact the root bridge for vlan 10. if we look at vlan 20 we're not told that it is the root bridge but we're told the root bridge id ends in 0 9 1 a so we assume that this should be switched to if we've configured everything correctly if we look at our interfaces we'll notice that we have a root port listed under gig 0-0 and of course with vlan 20 with switch 2 being the root bridge that makes complete sense if we look up under vlan 10 we'll notice that both gigs 0 0 and 0 slash 1 are in the forwarding state while back to vlan 20 gig 0 one is in the blocking state let's jump back to switch two and we'll verify things here look at our spanning tree instances and right away at the bottom we see that this verifies that this is the root bridge for vlan 20 as we would expect and similarly we see 927a we already know that's the bridge id for switch one and that's listed as the root bridge for vlan 10. so that all looks good that looks just as we would expect let's also say show run pipe to include spanning hyphen tree and this is going to give us our bridge priority values you can see both of those listed here for vlan 10 and for vlan 20. notice we have separate priority values for each vlan you might recall that the default priority value for spanning tree is 32 768 so pvst has altered those priority values we can see that the current priority value on switch 2 for vlan 10 is 28 672. let's jump to switch one and let's run the same command show run pipe to include spanning hyphen tree and we'll compare the value that we get on this side we can see that on switch one the priority for vlan 10 is 24 576. of course that priority is lower than the one we saw on switch two if we jump over there 28 672 on switch 2 24 576 on switch one this having a lower priority value means that of course this is going to be the root bridge for vlan 10 and we have the exact opposite scenario happening for vlan 20. we have the higher priority value on switch 1 and the lower priority value on switch 2 making switch to the root bridge for vlan 20. let's go here on switch 2. let's go under global configuration mode let's go under interface range gig zero slash zero through zero slash one those are both of our active trunk connections and what i want to do here is to simply shut down these interfaces so that we can see what happens with a secondary route bridge configured remember we've configured switch 1 as the primary route bridge for vlan 10 and the secondary route bridge for vlan 20. so if switch 2 goes down theoretically switch 1 should take over root bridge priorities for both vlans now that does take a little bit of time for that convergence to happen so this is a good time to talk about what these port states look like pvst plus uses the exact same port states as traditional spanning tree we start with 20 seconds of the blocking state and we start here in the blocking state in order to allow spanning tree to complete any convergence before any traffic is forwarded blocking ports do not forward frames and they do not learn mac addresses but they do receive bpdus from other switches so that they can learn about changes to the switching topology next the port will transition from blocking to listening for 15 seconds this is where the port can receive and also send bpdus so that it can participate in the election of the root bridge the root ports and the designated ports if this port is not selected as either a root port or a designated port it's going to eventually move back into the blocking state if the listening port though is selected as a root port or a designated port then it's going to transition to the learning state for 15 more seconds the port will continue to send and listen for bpd use and it will also begin to learn information it will begin to add information to its local mac address table and the final state is when the port moves from learning to forwarding and once this is complete then the port is fully functional it can send and receive bpd use it can learn mac addresses and it can forward frames any ports that are selected as root or designated ports are going to eventually transition into this forwarding state one more thing to point out here just some terminology is that the 15 seconds we see in the listening phase and the 15 seconds we see in the learning phase is what we call the forwarding delay which again is a total of 30 seconds by default in pvst plus now we can of course adjust this but that is the default value let's go back to switch one and let's see what's happened after we've shut down our interfaces on switch 2. let's say show spanning hyphen tree you see our multiple instances of spanning tree running notice now for vlan 20 we're told this is the root bridge if we go up for vlan 10 it's also the root bridge for vlan 10. so it looks like our secondary root bridge designation worked exactly as we would want it to switch one was able to take over root bridge responsibilities in the event of switch to failing so that completes our look at per vlan spanning tree plus in the next video i'll be looking at a variation of that called rapid pvst plus i hope you found this content useful and i want to thank you sincerely for watching

Channel: Charles Judd

Views: 1,110

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: xtywFJoVz4w

Length: 15min 31sec (931 seconds)

Published: Fri Aug 07 2020