Original subtitles

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What you’ll notice is Spanning Tree convergence is very quick

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and that’s because we’re running Rapid PVST+

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I didn’t make any changes on these switches from the Spanning Tree point of view.

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So the command sh run | include span shows the default config

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and notice the Spanning Tree used is Rapid PVST.

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in the output, it shows us RSTP or Rapid Spanning Tree

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but this is actually Rapid Per-VLAN Spanning Tree.

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We can change the mode of spanning Tree

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but before I do that just to make the point again.

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at the moment sh spanning-tree on switch 3

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shows that the root port is gigabit 0/1 this port.

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If I shut that port down and then type sh spanning-tree again

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what you’ll notice is gigabit 0/0 is the root port.

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convergence is very quick with Rapid Spanning Tree

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because it doesn’t use timers.

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So the max age and forward delay timers are not used for convergence.

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The switches send messages to each other

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with Rapid Spanning Tree to enable quick convergence.

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So now if we change the Spanning Tree type or mode to PVST

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we should see that Spanning Tree takes a lot longer to converge.

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So I’ll change that on all switches; switch 3, switch 4, switch 5.

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So back on switch 3 sh spanning-tree

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notice we can see that the switch is still learning

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which ports are the root port

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designated port or blocking port, when in the learning state

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traffic will be block

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user traffic will only be forwarded

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when ports transition to the forwarding state.

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At the moment you can see that the Spanning Tree

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protocol used are shown here is IEEE

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but once again you need to be careful

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because on Cisco switches even though it displays IEEE

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we're actually using PVST

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PVST is once again backward compatible

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so it will be able to talk to an 802.1D switch

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from another vendor as an example

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so we can see IEEE in the output here.

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So once again sh spanning-tree

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the root port, in this case, is now gigabit 0/0.

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So what happened because previously we had switch 1 as the root.

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It still has our command sh spanning-tree

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shows us that the switch is the root of the topology

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but gigabit 0/1 is not shown in the output here

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because I need to no shut that port.

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So conf t interface g0/1 no shut

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sh spanning-tree

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notice that port is a listening port

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gigabit 0/0 is a blocking port.

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So this port is blocking, this port is listening

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and what you’ll notice is it will take it a while to converge

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I'll put an IP address on this switch

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I’ll be waiting and then I'll demonstrate this again.

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Put an IP address on switch 1.

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I’ll no shut the interface

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so ping 10.1.1.1 it can ping itself

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switch 3 no shut the interface

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Ping 10.1.1.1 ping succeeds so I'll just do that again.

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Notice the ping from switch 3 to switch 1 succeeds.

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sh spanning-tree convergence has taken place

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because gigabit 0/1 is the root port

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and it's forwarding but now if I shut down gigabit 0/1

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and then try and ping switch 1, the port has gone down

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but pings are failing even though we have a redundant link.

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sh spanning-tree

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shows me that root port is still learning.

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Still learning, pings failed, PVST takes a long time to converge

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and take 30 seconds for that convergence take place.

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As you can see there, it's just happened

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sh spanning-tree

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shows us now that gigabit 0/1 is forwarding.

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But once again if I no shut gigabit 0/1

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and did the ping again, the ping would fail

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because it now needs to learn that this is the better path.

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sh spanning-tree

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notice the root port gigabit 0/1 is in the listening state.

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So we have listening, then we have learning

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and after while it should go to forwarding

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but that can take 30 seconds

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so it’s still learning, now it's gone to forwarding

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and now pings will succeed.

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So ports have different states.

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in a blocking state, user traffic is not forwarded

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the switch doesn’t learn MAC addresses based on frames received.

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This is a stable state for a port.

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A listening and learning port do not forward frames either

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listening ports don’t learn MAC address based on frames received

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In other words, they don’t update the MAC address table.

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A learning port does update the MAC address table.

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This is a temporary state or transitionary state

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while the switch has learned the topology.

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In a forwarding state frames are forwarded

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MAC addresses are learned and this is a stable state

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in other words, this is not a transitionary state

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this state will stay that way until there's change in the topology.

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A disabled port doesn’t receive6:01 PM 6/21/2017 frames, doesn’t forward frames

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doesn’t learn about MAC addresses on a port

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and this port will stand at state until you enable the port.

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Now if we change that to Rapid Spanning Tree

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So spanning-tree mode rapid-pvst

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what we should notice is that convergence takes place a lot quicker.

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I’ll only enable Rapid Spanning Tree on switch 1, 2 and 3

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sh spanning-tree rather sh spanning-tree

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shows us that the Spanning Tree mode now enabled

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is Rapid Per-VLAN spanning Tree

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we’ve got a path cost using gigabit 0/1

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so as you can see gigabit 0/1 is the root port.

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The switch can ping switch 1

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I'll shut that port down and when we do a ping again

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it instantly can ping switch 1

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even though we’ve just seen the interface go down in the output here

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because Spanning Tree converge is a lot quicker when using Rapid Spanning Tree.

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So the moral of the story is that in the real world

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you wanna use Rapid PVST rather than PVST.