Original subtitles

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In this topology, I have 2 switches which are configured to run PVST not Rapid PVST+

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but simply PVST I'll show you that config in a moment.

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Router 1 is connected to switch 1

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and router 2 is connected to switch 2

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and the routers are simply acting as edge devices or PCs in this topology.

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I’ve also got a hub connected to switch 1 and switch 2.

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Here’s switch 1, sh run | include span

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as you can see at the moment the switches configured for PVST

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I'll explain extended system IDs in more detail later

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but essentially it means that the priority of the switches

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based on the priority and VLAN number.

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So as an example, sh Spanning Tree

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this switch has a bridge ID consisting of the priority 32769

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which is the default of 32768+ the extended system ID of 1

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because we're looking at VLAN 1 and MAC address of the following.

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this switch is currently the root

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What I want you to see is that the Spanning Tree enabled here is IEEE

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so in this output, it looks like you're running 802.1D

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but actually, the switch is configured for Per-VLAN Spanning Tree.

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Per-VLAN Spanning Tree is compatible with 802.1D switches

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and therefore we can see IEEE in the output here.

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Here switch 2 sh run | include span

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switches configured to use PVST, extended system IDs are being used

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on this switch, the brIDge ID consist of the priority 32769

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which is 32768 the default + the VLAN number which is VLAN 1 in this example

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this is the MAC address of the switch.

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So we have 2 switches, one has this MAC address

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one has this MAC address, switch 1 has become the root of the Spanning Tree

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because it has a lower MAC address when compared to this switch.

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So because of the lower MAC address

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notice AC is lower than EA in hexadecimal

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switch 1 became the root of the Spanning Tree.

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What you’ll also notice is that on switch 1

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all ports are forwarding in the topology

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the ports that are currently connected are those ports

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and they're all forwarding on switch 2, however, port 1

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which is gigabit 0/0 is the root port

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and it's forwarding has path cost of 4, gigabit 0/1 is blocking

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or discarding to use the industry standard term.

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Port 2 is forwarding, port 3 is blocking.

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So this port is also blocking.

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Root switches forward on all ports.

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Before I show you how port status were determined

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let’s have a look at the BPDUs

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So I'll start capture on that link

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and what we can see here in Wireshark is a Spanning Tree BPDU.

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So it’s using 802.3 Ethernet frames

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notice the destination address is the well-known MAC address for Spanning Tree

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it is a multicast/broadcast address from this MAC address.

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Here’s switch 1

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notice the MAC address of the switch 00:11:c6:ac:dd:

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and we're currently looking at port 3 on the switch

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so notice dd00 but this is dd03

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because in Spanning Tree that’s the port that we're currently looking at.

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if we went and look at port 2 as an example

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notice the MAC address ends in 02

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we’ve got port 00 01 02 and 03

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so back in Wireshark here’s our capture sent out of port 3

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in Spanning Tree we can see the spanning 3 version

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so this is Spanning Tree 0

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because in this port its actually using 802.1D

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or the original version of Spanning Tree.

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The root identifier is 32768, there’s the VLAN number

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and there’s the MAC address of the switch, which we can see clearly here

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so notice root identifier, there’s the information of 32768 VLAN 1

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there is the MAC address of the switch.

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The root path cost is 0 because this switch is the root

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so there’s no cost to get to the root.

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There’s the port identifier and here are some timers used in Spanning Tree.

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Now when a switch boots up all ports are put into a blocking state

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they're then moved to other states based on timers in 802.1D

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When the Spanning Tree switch boots up all ports are put into the blocking state

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after 20 seconds called the max age timer ports moved

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to what’s called the listening state.

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if a switch is already up and you connect your cable to the port

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in other words, the links goes up

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it starts at the listening state

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ports will then move to the learning state

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based on the forward delay which is 15 seconds in duration

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and after 15 seconds ports transition from the learning state to the forwarding state.

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So an 802.1D or PVST it can take 50 seconds

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for ports to start forwarding on switches

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because they move from blocking to listening to learning to forwarding.

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Now in the listening state, they are sending BPDUs

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but not updating their MAC address tables.

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in the learning state, BPDUs are sent

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and the MAC address tables of switches are updated.

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So only if based on the Spanning Tree calculation

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it’s determined that a port can be opened

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a port is set to the forwarding state after 50 secs.

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when the switch comes up or typically if the switch is already up

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and you plug in a cable into that switch after 30 secs.

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The port will start forwarding.

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So in the BPDU we can see the max age timer

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and the forwarding delay timer.

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this is determined by the root bridge

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so on switch 1 which is the root bridge

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we can see that the hello timer

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in other words, BPDU hellos are sent out every 2 seconds

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the max age time is 20 seconds

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and the forwarding delay timer is 15 seconds

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and that’s what we see in the BPDUs as captured in our topology.