Across two routers

given
Lessons used	2.1 routers, 2.3 data centers and tracert, 3.3 VLSM
Log in	click laptop on the Topology tab (and r1, r2 for the routers)
Your answers	under the guide, in the lab app
The problem	Break it, next to Check task under the guide, breaks something on purpose

new to this? step by stepbeginner

Click laptop on the Topology tab. In the laptop tab: traceroute -n 192.168.2.10.
Click r1. In the r1 tab: show ip route. Do the same in an r2 tab. Fill in Your answers, under the guide.
Challenge: click Break it, next to Check task, then find out which router lost a route, and put it back.
Check task 3.1, under the guide.

How a packet finds its way

Asking for directions, one stop at a time

Imagine posting a letter to a town you’ve never been to. Your local post office doesn’t know every street in the country. It only knows letters for that region go to the big sorting office. The sorting office knows which local office to send it to, and that office knows the street. Nobody knows the whole way; each one knows the next stop.

Routers work like that. Each one has a routing table: a list of networks it knows how to reach, and for each one, where to send packets next. That where next is called the next hop.

in one sentence A router doesn’t know the whole path; its routing table says which next hop to send each network to.

How a router fills its routing table

How	What it means	In this lab
connected	a network on one of its own ports: it’s right there	r1 knows `192.168.1.0/24` because it has an address on it
static	a route someone typed in by hand	r1 was told 192.168.2.0/24: send it to r2
dynamic	routers telling each other what they know, with a routing protocol	not in this lab: it’s a later course

The networks between r1 and r2

The link between the two routers is 10.0.0.0/30: four addresses, two usable, one for each router. That’s the /30 from lesson 3.3, the right size for a cable between two routers.

Behind r2, the servers’ block 192.168.2.0/24 is split with VLSM into two /28s, one per server network: 192.168.2.0/28 for web and 192.168.2.48/28 for dns. r1 doesn’t need to know that. One route, 192.168.2.0/24 via r2, covers both, because both fit inside it.

Routes have to work both ways

A ping is two trips: there, and back. web’s answer goes to 192.168.1.50, the laptop, and every router on the way back needs a route for that network. If one doesn’t have it, the ping goes out fine and the answer is thrown away. From the laptop it looks just the same as if the packet never arrived.

in one sentence Every router on the way there needs a route to the destination, and every router on the way back needs a route to the source.

traceroute: who’s on the way?

In lesson 2.3 you tried tracert (on Windows) or traceroute (Mac and Linux) on your own computer. Here’s how it works.

Every packet carries a number called its TTL, time to live. Each router that passes the packet on takes 1 off it, and if it reaches 0, the router throws the packet away and sends a small message back to the sender: time exceeded, from the router’s own address. (That’s what stops a packet going round in circles for ever if the routes are wrong.)

traceroute uses this on purpose. It sends a packet with TTL 1: the first router drops it, and its message says who it is. Then TTL 2: the second router answers. And so on, until the destination itself answers. One line per stop.

in one sentence traceroute sends packets that run out of time one router further each time, so each router on the way says who it is.

2 show

A path and a routing table

From the laptop:

laptop:~# traceroute -n 192.168.2.10
traceroute to 192.168.2.10 (192.168.2.10), 30 hops max, 60 byte packets
 1  192.168.1.1  0.130 ms  0.021 ms  0.026 ms
 2  10.0.0.2  0.099 ms  0.040 ms  0.043 ms
 3  192.168.2.10  0.075 ms  0.051 ms  0.050 ms

Line	Who	Why that address
`1 192.168.1.1`	r1	its address on the side the packet came in: the home network
`2 10.0.0.2`	r2	its address on the link from r1
`3 192.168.2.10`	web	the destination: done

Each line has three times because traceroute tries each stop three times. -n means show numbers: don’t look up names for the addresses.

On r1, show ip route:

r1#show ip route
Codes: C - connected, L - local, S - static, O - OSPF, B - BGP, D - EIGRP
       ...
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        10.0.0.0/30 is directly connected, GigabitEthernet2
L        10.0.0.1/32 is directly connected, GigabitEthernet2
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet1
L        192.168.1.1/32 is directly connected, GigabitEthernet1
S     192.168.2.0/24 [1/0] via 10.0.0.2, 00:01:32, GigabitEthernet2

Letter	Meaning	Example
`C`	connected: a network on one of r1’s ports	`192.168.1.0/24` on GigabitEthernet1, the home network
`L`	local: r1’s own address on that port, as a `/32` (one address)	`192.168.1.1/32`
`S`	static: typed in	`192.168.2.0/24 via 10.0.0.2`: send it to r2

Gateway of last resort is not set means r1 has no default route: a packet for a network that isn’t in this table is thrown away. (Variably subnetted means the table has networks of more than one size in that block; you can skip those lines.)

3 follow along

Follow the path

Trace the path from the laptop to web, then to dns. Click laptop on the Topology tab. In the laptop tab:
run in the laptop tab
```
traceroute -n 192.168.2.10
traceroute -n 192.168.2.53
```
Both go through the same two routers. Only the last line differs.
Read r1’s routing table. Click r1 on the Topology tab. In the r1 tab:
run in the r1 tab
```
show ip route
```
Find the S line. Where does r1 send packets for 192.168.2.0/24?
Read r2’s. Click r2 on the Topology tab. In the r2 tab:
run in the r2 tab
```
show ip route
```
r2 has the two /28 server networks as C lines, the link to r1, and an S line for the way back to the home network, 192.168.1.0/24 via 10.0.0.1.

Fill in the follow-along answers in Your answers, under the guide:

editYour answersunder the guide

Where r1 sends packets for 192.168.2.0/24: 10.0.0.2
How many hops traceroute counts to web: 3

in one sentence traceroute shows the path from outside; show ip route on each router shows why it goes that way.

4 challenge

Something’s broken

Break the network: click Break it, next to Check task 3.1 under the guide.

It changes one router’s configuration. Don’t go looking for what it changed: find the problem the way you would on a real network, from the laptop first. In the laptop tab:

laptop:~# traceroute -n -m 5 192.168.2.10

(-m 5 means give up after 5 stops, so you don’t wait for 30.) Then:

work out which router lost a route, and answer Which router was missing a route with r1 or r2 in Your answers;
put the route back, on that router;
click Check task 3.1.

hint

traceroute gets an answer from r1 and then only *: nobody answers. Can the laptop ping r1’s far side, 10.0.0.1? And r2’s near side, 10.0.0.2?

hint

Remember, routes have to work both ways. r2’s answers have to come back to 192.168.1.50. Look at show ip route on r2: does it know where 192.168.1.0/24 is?

hint

A static route in configuration mode is ip route <network> <mask> <next hop>. The next hop is the router on the other end of the link.

show the solution

r2 lost its route back to the home network: its show ip route has no S line any more. Packets from the laptop still reach r2, and even web, but every answer (the ping replies, and r2’s own time exceeded messages) is for 192.168.1.50, and r2 has no idea where that is. So it throws them away, and the laptop sees nothing after r1.

Put it back on r2, in the r2 tab:

r2#configure terminal
r2(config)#ip route 192.168.1.0 255.255.255.0 10.0.0.1
r2(config)#end
r2#show ip route
...
S     192.168.1.0/24 [1/0] via 10.0.0.1, 00:00:03, GigabitEthernet1

and in Your answers:

Which router was missing a route: r2

traceroute -n 192.168.2.10 shows all three stops again. This is one of the most common faults there is: a route on the way there and none on the way back. When the first stop answers and everything after it is silent, look at the return path.

objective

How a packet finds its way

Asking for directions, one stop at a time

How a router fills its routing table

The networks between r1 and r2

Routes have to work both ways

traceroute: who’s on the way?

A path and a routing table

Follow the path

Something’s broken

5 check: run it, then prove it