Original subtitles

In this example, we’ve got 2 switches and 4 routers

which are acting as PCs in the topology

we also got a hub which I'm going to use to capture traffic using Wireshark.

These devices have just booted up so that they have no configuration.

on the first switch, I’ll give this switch a hostname of S1

and on the second switch give it a hostname of S2.

sh vlan at the moment we only have 1 Ethernet VLAN configured VLAN 1.

All ports on the switch are configured in that VLAN

Cisco switches also have some older VLANs by default 1002, 1003, 1004 and 1005 10

which were used previously for FDDI and token ring 11

those VLANs are not used anymore today. 12

Switch 2 shows something similar so sh vlan brief 13

we have VLAN 1 all ports are configured in VLAN 1. 14

Here’s router 1 which is acting as our PC 15

I’m going to configure the FastEthernet interface 16

with an IP address of 10.1.1.1/24 mask 17

and I’ll no shut the interface on router 2 18

do something similar 19

no shut the interface given an IP address of 10.1.1.2 20

so without any further configuration 21

we should be able to ping between router 1 and router 2 22

notice router 2 can ping router 1 and router 1 can ping router 2. 23

The switch just has a default config 24

all ports on the switches are in VLAN 1 25

we can see that by using the command sh int g0/0 switchport 26

this port gigabit 0/0 is using administrative mode dynamic auto 27

operation mode at the moment is static access 28

negotiation of trunking is on, in other words DTP is enabled 29

but at the moment because there is no switch to negotiate with 30

the port is in VLAN 1, the default VLAN. 31

Now we can see the traffic by doing a capture using Wireshark. 32

Various traffic is seen here we’ve got a Spanning Tree BPDU being sent 33

by the switch, we can see Cisco discovery protocol messages 34

being sent by the switch as well 35

showing the switch port, the platform and other information. 36

But what I’d like you to see is all traffic is Ethernet frames. 37

There is no 802.1Q tagging on this frames. 38

Let’s do a search for ICMP traffic. 39

At the moment there's is no ICMP traffic. 40

I'll get router 1 to ping router 2 and notice we see the ICMP traffic 41

what you’ll notice is we have the source MAC address of router 1 pinging router 2 42

protocol type is IPv4 source IPv4 address is 10.1.1.1 43

destination IP address is 10.1.1.2 44

it’s a standard Ethernet frame encapsulating IP and ICMP. 45

In this case it’s an echo and the reply is in echo reply. 46

There is no Ethernet tagging on this frames at all. 47

It's a standard Ethernet frame, there is no concept of VLAN either 48

no VLAN information is transmitted on the frames. 49

Even the other frames such as CDP don’t show any VLAN information whatsoever. 50

I'll stop that capture. 51

this is an access port and this is an access port by default 52

the same will be true on switch 2 53

I'll configure router 3 with an IP address of 10.1.1.3 54

no shut the interface and I'll do something similar on router 4 55

no shut IP address 10.1.1.4 56

Can router 4 ping routers 3 acting as host 3? 57

Yes it can. 58

Start capture here. 59

We can see various traffic types being captured 60

including once again CDP on router 3, I'll ping router 4 ping succeeds. 61

I’ll filter for ICMP 62

and once again you can see that this is a standard Ethernet frame. 63

There is no information about VLANs contained in this frames. 64

Just standard Ethernet frames. 65

So the PCs in the topology are unaware of any VLAN information. 66

they're sending untagged frames 67

in other words, standard Ethernet frames to the switches 68

and the switches are sending untagged frames to the PCs. 69

Now let’s concentrate on switch 1 for the moment 70

I’m gonna set the VTP mode of this device to transparent 71

which means it won’t try and synchronize VLAN information with the other switch. 72

sh vlan once again show us that we have 1 Ethernet VLAN configured. 73

Let's create VLAN 2, sh vlan brief 74

we now have 2 Ethernet VLANs but no ports are configured in VLAN 2.