Original subtitles

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At the moment in this topology switch 2 is the root

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and we can prove that by using sh spanning-tree

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and notice the local switch is the root for VLAN 1 in the topology.

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No other VLANs have been enabled

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so the only Ethernet VLAN existing at the moment is VLAN 1

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and all ports are VLAN 1 on all the switches.

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When looking at the BPDUs

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we can see that the switch is advertising a root path cost of 0

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and the bridge ID and root identifier are the same.

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On switch 1 when we capture traffic here

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it's advertising a root path cost of 4

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and you can see that the bridge identifier is different to the root identifier

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So on switch 1, let’s change the spanning-tree vlan 1 root

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so I'll change the switch to become the root and in this case, it’ll be the primary root

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that will drop the priority of the switch.

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previously the switch was using a default priority

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which we can see in the Wireshark capture

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it was 32768 + the VLAN number but now sh spanning-tree

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shows us that the switch priority has been reduced

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to a value lower than any current root bridge.

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So the switches now become the root of the Spanning Tree topology.

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Bridge MAC address and root MAC address is the same.

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So these are our first Wireshark capture

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and if I scroll down to the latest advertisements.

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Notice the root path cost is 0.

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Bridge ID and root identifier are the same, the priorities have been reduced.

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So this switch is root with the lower priority.

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You can see that the bridge priority is 24576

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extended system ID is 1 bacause this is VLAN 1

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and the MAC address of the switch is set to the following.

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So once again, if we look at the Wireshark capture

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this is capture 114 at the moment.

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Previously the root identifier and bridge identifier were different.

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Now, what about switch 2 previously this switch was the root.

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So notice on capture 1, this switch previously had the root path cost of 0

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but scrolling down through the captures

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you can now see that it has a root path cost of 4.

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Its local bridge identifier has a higher priority.

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So this switch is no longer the Spanning Tree root.

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Previously on the switch 3, this was the root port.

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So here's the output you can see that gigabit 0/0

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was the root port on switch 3.

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What is it now? So sh spanning-tree

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notice gigabit 0/1 is the root port at the moment.

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so previously on switch 3 this was the root port

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but the root port is now changed to this port

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because this switch is now the Spanning Tree root.

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the path cost using this link is 4

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where's now the path cost using this link would be 4 + 4

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in other words 8 to get to the root bridge.

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Spanning tree topologies can dynamically change

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so as an example, if on switch 3 has shut down gigabit 0/1

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and type sh spanning-tree

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What we can see now is that gigabit 0/0 is now the root port

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as this port is no longer available.

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If I no shut that interface and type sh spanning-tree

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and see the interface is coming up.

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What you can see now is that gigabit 0/1 has become the root port once again

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where's this port is now blocking to stop a Spanning Tree loop.