Original subtitles

So let’s do a test

what happens if we move one of these ports into a different VLAN?

Now earlier when I was doing this test I had some problem in GNS3.

So what I’m gonna do is I’m gonna shut g0/2 down and g0/3.

So the only interfaces that are now up are 0/0 and 0/1

in this topology, just to start with a simple network to make a point.

So those interfaces are up, up the other interfaces are down.

What I'll also do actually is to shutdown any other interfaces

to make sure the things converge quicker. 10

So I'll shut down that range and this range of interfaces 11

sh ip int brief also shut down gigabit 3/0 - 3 12

So sh ip interface brief 13

All our interfaces are shutdown except for those 2 interfaces 14

so gigabit 0/0, gigabit 0/1 15

Is router 1 able to ping router 2? 16

Yes it is but if we put this interface gigabit 0/1 into VLAN 2 17

what will happen with the pings? 18

So switchport access vlan 2 19

now before I present here, notice the ping succeeds I'll repeat this 100 times. 20

And then hit enter on the switchport access VLAN command. 21

Notice the pings are starting to time out 22

so as soon as I moved the port from 1 VLAN to another 23

the devices are not able to communicate with each other. 24

sh spanning tree shows that 25

gigabit 0/1 is still in the learning phase of Spanning Tree 26

so we'll wait for a while for Spanning Tree to converge and then do the test again. 27

But what I’d like you to see is this, 2 devices were in the same subnet 28

and at the same VLAN and they were able to ping each other 29

as soon as we moved one port to different VLAN 30

they were no longer able to ping each other. 31

Spanning Tree is forwarding on the VLAN 2 on port gigabit 0/1 32

it’s also forwarding on gigabit 0/0 on VLAN 1. 33

So if we use the sh spanning tree summary command 34

we can see that VLAN 1 is forwarding 35

VLAN 2 is forwarding, there are no blocking ports 36

yet router 1 is not able to ping router 2 37

and router 2 is not able to ping router 1. 38

Because they are in separate VLANs. 39

Is router 2 aware that it’s in a separate VLAN? 40

The answer is no because no tagging information 41

or no VLAN information is gonna being sent on this port it’s an access port. 42

So we just a standard Ethernet frames. 43

There is no VLAN port information transmitted 44

on any of the frames going out on that port. 45

Now to prove this. 46

Let’s add some IP addresses into switch 1 47

so interface vlan 1 ip address 10.1.1. 48

Let’s make it 254 and then interface vlan 2 ip address 10.1.2.254 49

for the mask, now need to no shut both of those 50

so go back to vlan 1 and no shut it. 51

So these are layer 3 Switch Virtual Interfaces on the switch. 52

we are basically creating a layer 3 IP address 53

on the switch for the relevant VLAN 54

so as an example, this switch can ping router 1 on VLAN 1 55

it lost the first ping because of ARP 56

but switch 1 can ping router 1 57

it can’t ping router 2 because router 2 58

needs to be configured with the right IP address for VLAN 2 59

but before I do that notice when I do a capture on that port 60

traffic from the switch to the router is untagged 61

it's a standard Ethernet frame IP traffic 62

there is no tagging at all and as a last test 63

what I’ll do is configure router 2 64

so interface f0/0 ip address 10.1.2.2 65

so I’ve moved it from 1 subnet to another. 66

Ping 2.2 the switch can ping router 2 on this port. 67

Is the traffic tagged? So do the ping again. 68

Filter for ICMP traffic, notice there is no tagging information at all. 69

It’s just standard Ethernet. 70

So what is the summary of this test? 71

Their PCs in the topology are unaware of VLAN traffic 72

these are access ports or untagged ports. 73

In other words 802.1Q tagging is not used on these ports. 74

Can router 1 ping router 2? 75

so cannot ping 10.1.2.2 76

at the moment it won’t be able to 77

because the routers don’t have default routes configured. 78

So I’m gonna turn off IP routing 79

on this routers to turn them into PCs with dumb devices 80

and type IP default gateway 81

and in this case the default gateway router 2 82

will be this IP address on router 1 83

no IP routing that’s a command that turns an expensive router 84

into a dumb device IP default gateway 10.1.1.254 85

so router 1 has a default gateway configured. 86

Can it ping its default gateway? 87

Yes it can, can it ping router 2? 88

At the moment it can and the reason why is that 89

on these switches IP routing is configured by default. 90

However, if I type no IP routing which is true on a lot of switches. 91

The pings will not succeed because the switch is not doing Inter-VLAN routing. 92

To enable Inter-VLAN routing on a layer 3 switch such as this 93

or a physical switch you need the IP routing command 94

to route between the VLANs. 95

So to prove this on router 2 96

I’ll do a debug ip icmp so we can see if ICMP traffic is getting to the router. 97

Do the ping again and notice there are the echo replies 98

we can do something similar on router 1 and there are the echo replies. 99

debug ip packet will give us low information 100

I repeat this only once so we'll send 1 ping 101

which succeeded over there 102

and what you can see is the packet was routed out of the router 103

and sent to the destination as an IP packet. 104

So in other words, this end devices connected to a switch 105

are unaware that the switch is using VLANs. 106

The configuration of the switches as follows. 107

sh run int g0/0 108

that port is using default config which 109

which means that it’s an access port in VLAN 1 110

gigabit 0/2 is an access port in VLAN 2. 111

IP routing is enabled and we’ve configured layer 3 IP addresses 112

on the 2 VLANs to allow the switch to route between their 2 VLANs 113

configured which in turn allows router 1 and router 2 114

to communicate with each other. 115

Now if router 1 and router 3 are put into the same VLAN 116

and router 2 and router 4 are put into the same VLAN. 117

That kind of information needs to be communicated 118

from 1 switch to another using 802.1Q 119

So this port needs to be configured as a trunk port 120

so let’s prove that and then configure it.