Original subtitles

1

In the real world, you would never plug a hub between 2 switches

2

because it will degrade your network dramatically.

3

In this example, we’ve only got the hub in the topology

4

so that we can capture traffic using Wireshark

5

you generally have a direct link between the 2 switches as shown here.

6

In our topology, I've shut that force down

7

to force all traffic to go via the hub

8

so that we can see what’s taking place

9

if I enable this port Spanning Tree will block one of these ports

10

so we won’t be able to see the traffic.

11

So at the moment PC 1 can ping PC 3

12

PC 2 can ping PC 4 but they can't ping each other.

13

So let’s setup switch 1 as our Inter-VLAN router.

14

So on switch 1 sh ip int brief

15

at the moment we have an IP address configured on VLAN 1 and VLAN 2.

16

Now router 1 and router 3

17

are still using the IP addressing of VLAN 1 which is bad practice

18

we can see that so on router 1 sh ip int brief

19

notice the IP address of this device is 10.1.1.1

20

so I'll change that to put it into VLAN 11, so IP 10.1.11.1

21

Just good practice you could keep those IP addresses

22

but it will get confusing when you set up a management VLAN.

23

So I'll just configure it correctly at this point

24

On router 3 currently the IP address is 10.1.1.3

25

so I'll change that to 10.1.11.3 and set up the default gateway 10.1.11.254

26

and then we need to configure VLAN 11 on switch 1

27

so interface VLAN 11, IP address 10.1.11.254

28

I showed that SVI or Switch Virtual Interfaces is up which it is.

29

So can router 1 ping router 3?

30

Just do a test, ping 10.1.11.3 yes, it can.

31

Can it ping router 2?

32

router 2 using this IP address 10.1.2.2 yes, it can.

33

Can it ping router 4 acting as PC 2?

34

So that’s 2.4

35

Let’s just do some troubleshooting

36

so first thing to confirm is, let’s check the interface

37

that interface looks right, let's ping 11.1

38

why is the ping not working?

39

it’s probably because I haven't set up a default gateway on router 4.

40

Let’s confirm that sh ip route

41

there are no default gateways configured, so I'll turn off the IP routing.

42

Before I configure the default gateway

43

let me show you a test that we can do to prove what’s happening.

44

So on router 1, I’m pinging router 4

45

and this debug, debug ip icmp shows us that the traffic hits router 4

46

but it’s not getting back.

47

So we can see that the traffic traverses

48

the network gets here but doesn’t get back again

49

and that allows me to see that

50

it’s because my default gateway is not configured.

51

So IP default gateway 10.1.2.254 on router 4 because it’s in VLAN 2.

52

So once again.

53

This device is in VLAN 11.

54

This device is in VLAN 11, these 2 devices are on VLAN 2.

55

So can router 1 ping router 4? Yes, it can.

56

So let’s have a look at the 802.1Q frames sent.

57

Here’s router 1 pinging router 4

58

and let’s look at the ICMP

59

what you’ll notice is the messages from 10.1.11.1 to 10.1.2.1

60

are actually being sent on VLAN 2

61

and the return traffic is being sent on VLAN 2

62

and that’s because the traffic is being sent from router 1 to switch 1

63

and switch 1 has already doing the Inter-VLAN routing

64

and then forwarding the traffic to switch 2 on VLAN 2

65

so it’s already changing the VLAN information to the VLAN 2

66

when forwarding it to the other switch.

67

when the traffic goes from router 4 to router 1

68

the traffic it's sent here is an untagged frame

69

and then here it's tag as VLAN 2

70

when it hits the switch which is acting as the default gateway

71

it has the Inter-VLAN routing and then forwards it out of its port

72

because it’s in VLAN 11 and the traffic is going to the device in VLAN 11.

73

So hence here we don’t actually see the tagged frames

74

but we can see that by adding a router on a stick to do the Inter-VLAN routing.

75

So what I’ll do is remove the Inter-VLAN routing on switch 1

76

and configure this device as the routing device in the topology

77

and then what we should see is we should see the tag frames hit this router

78

and then the tagging will be changed.