Original subtitles

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So switch 1 we're gonna shut this interface down

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to force traffic to go via the hub

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So interface gigabit 0/2 shut it down

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sh in g0/3 switchport

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this port gigabit 0/3 is acting as a trunk

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using 802.1Q all VLANs are allowed.

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So let’s do a capture on the hub, were receiving PVST information

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and notice the difference here's an 802.1Q header

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so we have Ethernet but notice the type is not IPv4

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the type is 802.1Q, so the type is 0x8100 rather than 0x0800 for IP.

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so 802.1Q frame, we can see here

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that the VLAN ID for this PVST message is VLAN 1

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Per-VLAN Spanning Tree sends what are called BPDUs on every VLAN

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so here we can see some Spanning Tree information

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and as we scroll down

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we can see that as an example CDP and VTP messages

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or in this case, DTP is sent as an untagged frame.

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So we can see some DTP information arriving here.

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And that will continue on.

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So let’s do a ping from router 2 acting as PC 2 to router 4

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and see if we can see why the frames are not permitted.

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So ICMP is showing nothing, so we can see a broadcast here

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this is an ARP message saying who has 10.1.2.4

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so in this ARP message, I'll just stop that capture for a moment.

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Here’s the ARP you can see it's an Ethernet 2 frame

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with the broadcast so the destination is broadcast.

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Source MAC address is the MAC address of router 2.

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We can see that by looking at the interface

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so sh int f0/0 notice there’s the MAC address of the router

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and they're already shown in the Wireshark capture

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but notice the type is once again 802.1Q and the VLAN is set to VLAN 2.

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So in other words, the router is sending the frame untagged to the switch.

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But when it go across this link it's going as a tagged frame.

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The switch is setting the VLAN tag to 2

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because it arrived on this port which is in VLAN 2.

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I’m just reset that capture

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and what I want to show you is on this port

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it's gonna show us untagged, so it's standard Ethernet

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but on this one, it's gonna show us tagged

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because the switch is tagging the frame.

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So I'll do the ping again

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now this is the frame to the switch from the router

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and we'll the switch for ARP, you can see this, the ARP traffic

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from the router to a broadcast address it's untagged, this is the type for ARP.

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There is no 802.1Q header here at all.

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But when we look for ARP on the link between these switches

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notice you can see the 802.1Q tagged set to 2.

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So the traffic is arriving here as untagged

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and the switch is tagging it to send the frame to switch 2.

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The problem here is when the traffic hits switch 2

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switch 2 is not configured with trunking

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this is just an access port in VLAN 1.

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So that traffic will be sent to this port

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but not out of this port which is in VLAN 2.

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So on switch 2 let’s see if we can see that.

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sh int trunk

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At the moment no interfaces are trunking

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so sh int g0/3

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and let’s put switchport at the end

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so sh interfaces g0/3 switchport

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this interface gigabit 0/3 is enabled

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it's set for negotiation of trunking. So it’s in VLAN 1.

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This port belongs to VLAN 1, it’s an access port, no trunking is enabled

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so traffic from router 2 is simply gonna be sent out of this port.

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So let’s do a capture there to prove that

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and then we'll do a capture on this port

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to check if any traffic arrives on this port.

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So there's the Wireshark capture

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do the ping again on router 2

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I’ll do a filter for ARP

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and notice there's the broadcast traffic from 10.1.2.2

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which is router 2 asking for the MAC address of 10.1.2.4

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So router 2 is asking for the MAC address of router 4 but it never receives it.

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So what you'll also notice here is there is no 8021.Q tag.

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So the frame was sent untagged here

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it was sent tagged cross here

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arrived here but this port was configured as an access port in VLAN 1.

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So the traffic was simply copied out on this port in VLAN 1 and with no tag.

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The traffic never arrived in this port.

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So let’s prove that do a capture here.

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I’ll do the ping again, and try and filter for ARP

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and what you’ll notice is there's no ARP traffic

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because the traffic is arriving on an access port.

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Port 1 it’s only gonna be sent out of this port which is port 1.

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It’s not gonna set out of this port at all.

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so let’s stop all captures

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and then what we'll do now is configure this port to be a trunk port

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and this port to be a trunk port

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but we'll leave this port shutdown for the moment

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so that we can see the traffic being captured.