Original subtitles

1

Now ping is great for testing as an example

2

if I ping 10.1.1.2 the ping succeeds

3

the reason the first ping failed is due to ARP

4

the router needs to ARP for the MAC address of router 2

5

but notice if I ping 10.1.2.2 which is the IP address of router 3

6

the pings fail because that route is not in the routing table.

7

10.1.2.0 is not in the local router's routing table.

8

10.1.1.0 is in the local routing table.

9

so the router can route appropriately

10

now debugs are very useful for testing and troubleshooting

11

and in this example, I’m gonna use a command debug ip icmp

12

on both router 2 and router 3 just to prove that traffic is arriving

13

as expected on the correct routers.

14

so ping 10.1.1.2 this IP address

15

notice we see the ICMP echo replies sent back to 10.1.1.1

16

let's do that so 10.1.2.2 the traffic doesn’t reach this router

17

we don’t see any outputs on the screen of router 3

18

where’s just to do it again to 10.1.1.2

19

we see the debugs because the traffic is being sent from router 1 to router 2

20

and is being received by router 2 which in turn is replying back.

21

So on router 1, we can use the command debug ip packet

22

be careful with this command in a live environment.

23

In a live environment, you want to use an access control list

24

to limit the traffic that you are debugging in a production

25

or live environment, you can kill your router if you run this command.

26

But in this live environment, it’s ok to do it.

27

so from a testing and troubleshooting point of view

28

lets work through logic

29

on router 1, we try to ping 10.1.2.2

30

we're not getting a reply we run a debug IP ICMP

31

on router 3 to see if the traffic is actually arriving at router 3

32

and it's not, that’s useful because pings send traffic

33

and then expect to respond

34

and this would allows to check whether the problem is on the path R1 to R3

35

or whether the problem is on the return R3 to R1

36

so this is a great command for testing

37

so ping 10.1.2.2 notice in the output we're getting unroutable

38

the local router is telling us that the source IP address 10.1.1.1

39

going to the destination 10.1.2.2 is unroutable

40

so it doesn’t know where to forward the traffic.

41

another great thing in the output here

42

is that we know which interface the router is using for sending packets

43

typically a router uses the outgoing interface

44

as the source in packets and you can see that here.

45

so the source of the ping is 10.1.1.1

46

because that’s the outgoing interface to get to a destination

47

such as 10.1.2.2

48

as an example, if we ping 10.1.1.2 the ping succeed

49

we get quite a lot of output

50

so scrolling back, there’s the ping, they are the success messages

51

we can see that the source IP address is 10.1.1.1

52

destination is 10.1.1.2 this IP address

53

and this was routed via the Forwarding Information Base or FIB out of F0/0

54

so the packet was sent

55

and then we received a reply from 10.1.1.2 going to 10.1.1.1

56

so here we can see that the packet was forwarded out of the router

57

or sent by the router and here we can see a replied back from router 2.

58

so a very useful command but once again be careful using it in production

59

sh ip route

60

shows me that I don’t have a route in the routing table for 10.1.2.0

61

and I also don’t have a gateway of last resort

62

in other words, I don’t have a default gateway

63

or default route configured on this router.

64

So the router doesn’t know what to do with the traffic

65

and hence when we ping the router tells us that the traffic is unroutable.

66

The router doesn’t know where to forward the traffic

67

so we need to configure static routes.