Lab 12: Routed Two-Branch Network

Companion lab to Week 13 — Understanding Routers. You will build the same topology used in the lecture animation, then extend it to three routers with a simulated internet link.

Not marked. No file submission required. Save your .pkt file as you work. All router and switch configuration must be done via the CLI tab — the GUI Config tab is off-limits.

Overview

Part	Session	Mode	What you do	What you prove
1	1	Solo	Build the topology in Packet Tracer — devices, serial modules, cables, static PC IPs.	All link lights green; PCs have correct static IPs.
2	1	Solo	Configure R1 interfaces (Fa0/0 LAN + Se0/0/0 serial) via CLI.	`show ip interface brief` shows both interfaces up/up.
3	1	Solo	Configure R2 interfaces (Fa0/0 LAN + Se0/0/0 serial) via CLI.	`show ip interface brief` shows both interfaces up/up.
4	1	Solo	Add a static route on R1 to reach Branch B, and on R2 to reach Branch A.	`show ip route` shows static entries on both routers.
5	1	Solo	Run a full branch-to-branch connectivity checklist.	PC-A1 can ping PC-B1 and vice versa.
6	2	Solo	Add R3 (ISP router), a second serial link from R1, and a simulated internet LAN with a web server.	R3 interfaces are up/up; Web-Server has a static IP.
7	2	Solo	Configure R3 interfaces and static routes. Replace specific remote routes on R1 and R2 with default routes pointing toward R3.	`show ip route` shows a default route (`S*`) on R1 and R2.
8	2	Solo	Run an extended connectivity checklist — branch-to-branch and branch-to-internet.	Every PC can ping the Web-Server at `10.0.0.10`.
9	3	Pairs	Take turns introducing and diagnosing 4 deliberate routing faults.	Can identify the symptom, run the right `show` command, and apply the fix.

Prerequisites

Cisco Packet Tracer 8.x installed and signed in.
Completion of Labs 1–11 — in particular Lab 11 (router sub-interfaces).
Your completed Lab 11 .pkt file is not required for this lab — this is a fresh topology.
All router configuration must be done via the CLI tab — not the GUI Config tab.
For Part 9 (pairs): both students need their own working copy of the finished Part 8 network.

IOS CLI Quick Reference

Use this table while working through all three sessions. It covers every command you will need.

Router Interface Commands

What you want	Command
Enter privileged mode	`enable`
Enter global config	`configure terminal`
Set hostname	`hostname R1`
Select a physical interface	`interface Fa0/0` or `interface Se0/0/0`
Assign an IP address	`ip address 192.168.1.1 255.255.255.0`
Bring the interface up	`no shutdown`
Set serial clock rate (DCE end only)	`clock rate 64000`
Shut down an interface	`shutdown`
Exit one level	`exit`
Exit to privileged mode	`end`
Save running config	`write memory`

Router Static Routing Commands

What you want	Command
Add a specific static route	`ip route 192.168.0.0 255.255.255.0 200.100.100.254`
Add a default route (gateway of last resort)	`ip route 0.0.0.0 0.0.0.0 172.16.0.2`
Remove a static route	`no ip route 192.168.0.0 255.255.255.0 200.100.100.254`

Verification Commands

What you want	Command
Show all interface statuses + IPs	`show ip interface brief`
Show the full routing table	`show ip route`
Show only static routes	`show ip route static`
Show running configuration	`show running-config`
Show one interface in detail	`show interfaces Se0/0/0`

PC Commands

What you want	Command
Show IP configuration	`ipconfig`
Test reachability	`ping 192.168.0.10`
Trace the route hop-by-hop	`tracert 10.0.0.10`

Network Plan

TechCo NZ has opened a second branch office (Branch B) and wants a connection to the internet via an ISP router. The address scheme matches the lecture animation from Week 13 — if something looks familiar, it should!

Network	Address	Subnet Mask	Used for
LAN–A	`192.168.1.0`	`255.255.255.0`	Branch A — PCs behind R1
WAN R1↔R2	`200.100.100.0`	`255.255.255.0`	Serial link between R1 and R2
LAN–B	`192.168.0.0`	`255.255.255.0`	Branch B — PCs behind R2
WAN R1↔R3 (Session 2)	`172.16.0.0`	`255.255.255.252` (/30)	Serial link between R1 and R3 (ISP)
Internet LAN (Session 2)	`10.0.0.0`	`255.255.255.0`	Simulated internet — Web-Server behind R3

Interface Plan

Device	Interface	IP Address	Subnet Mask	Notes
R1	`Fa0/0`	`192.168.1.1`	`255.255.255.0`	Default gateway for Branch A PCs
R1	`Se0/0/0`	`200.100.100.1`	`255.255.255.0`	DCE end — must set `clock rate`
R1 (Session 2)	`Se0/0/1`	`172.16.0.1`	`255.255.255.252`	DCE end to R3 — must set `clock rate`
R2	`Fa0/0`	`192.168.0.1`	`255.255.255.0`	Default gateway for Branch B PCs
R2	`Se0/0/0`	`200.100.100.254`	`255.255.255.0`	DTE end — no clock rate needed
R3 (Session 2)	`Se0/0/0`	`172.16.0.2`	`255.255.255.252`	DTE end — no clock rate needed
R3 (Session 2)	`Fa0/0`	`10.0.0.1`	`255.255.255.0`	Gateway for internet LAN
PC-A1	NIC	`192.168.1.10`	`255.255.255.0`	Gateway: `192.168.1.1`
PC-A2	NIC	`192.168.1.11`	`255.255.255.0`	Gateway: `192.168.1.1`
PC-B1	NIC	`192.168.0.10`	`255.255.255.0`	Gateway: `192.168.0.1`
PC-B2	NIC	`192.168.0.11`	`255.255.255.0`	Gateway: `192.168.0.1`
Web-Server (Session 2)	NIC	`10.0.0.10`	`255.255.255.0`	Gateway: `10.0.0.1`

Session 1 — Build the Two-Router Topology

Part 1 — Build the Topology

Time: ~25 min • Save as: Lab12_<yourname>.pkt

1.1 Add devices

Open Packet Tracer → File → New.
From the Network Devices panel, add:
- 2 × 2960-24TT Switch — rename: SW-A and SW-B
- 2 × 2811 Router — rename: R1 and R2
- 2 × PC — rename: PC-A1, PC-A2
- 2 × PC — rename: PC-B1, PC-B2

1.2 Add serial (WIC-2T) modules to the routers

The 2811 does not have serial ports by default — you need to add a WIC-2T module.

Important. You must power the router OFF before inserting a module. Forgetting this is the single most common mistake in this step.

Click on R1 → go to the Physical tab.
Click the power button on the router image to turn it OFF. The green LED goes dark.
In the Modules panel on the left, find WIC-2T.
Drag the WIC-2T module and drop it into an empty slot on the router image. You will see two serial ports appear: Se0/0/0 and Se0/0/1.
Click the power button again to turn R1 back ON.
Repeat steps 1–5 for R2. R2 only needs one serial port, but WIC-2T is fine.

1.3 Connect cables

Use the cable types shown below. Serial cables have two ends — the DCE end connects to R1, the DTE end connects to R2.

Cable type	From device / port	To device / port
Copper Straight-Through	PC-A1	SW-A — `Fa0/1`
Copper Straight-Through	PC-A2	SW-A — `Fa0/2`
Copper Straight-Through	R1 `Fa0/0`	SW-A — `Fa0/24`
Copper Straight-Through	PC-B1	SW-B — `Fa0/1`
Copper Straight-Through	PC-B2	SW-B — `Fa0/2`
Copper Straight-Through	R2 `Fa0/0`	SW-B — `Fa0/24`
Serial DCE	R1 `Se0/0/0` — DCE end	R2 `Se0/0/0` — DTE end

Wait for all link lights to turn green. The serial link between R1 and R2 may stay red until interfaces are configured — that is normal.

Choosing the right serial end. When you select “Serial DCE” cable in Packet Tracer, the first port you click gets the DCE end. Click R1’s Se0/0/0 first, then click R2’s Se0/0/0. If you did it the other way round, remove the cable and redo it — the clock rate must be on R1.

1.4 Configure static IPs on PCs

Click each PC → Desktop → IP Configuration → Static and enter the values from the interface plan:

PC	IP Address	Subnet Mask	Default Gateway
PC-A1	`192.168.1.10`	`255.255.255.0`	`192.168.1.1`
PC-A2	`192.168.1.11`	`255.255.255.0`	`192.168.1.1`
PC-B1	`192.168.0.10`	`255.255.255.0`	`192.168.0.1`
PC-B2	`192.168.0.11`	`255.255.255.0`	`192.168.0.1`

Part 2 — Configure R1

Time: ~20 min • Click R1 → CLI. Press Enter to dismiss the banner.

2.1 Set hostname and configure the LAN interface (Fa0/0)

Router> enable
Router# configure terminal
Router(config)# hostname R1

R1(config)# interface Fa0/0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit

You should see: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up

2.2 Configure the serial WAN interface (Se0/0/0) — DCE

Clock rate. Because R1 has the DCE end of the serial cable, it must provide the clock signal. The clock rate command tells R1 how fast to clock the link. Without it the serial link stays down.

R1(config)# interface Se0/0/0
R1(config-if)# ip address 200.100.100.1 255.255.255.0
R1(config-if)# clock rate 64000
R1(config-if)# no shutdown
R1(config-if)# exit

R1(config)# end
R1# write memory

2.3 Verify R1 interfaces

R1# show ip interface brief

Expected output (after R2 is also configured):

Interface              IP-Address       OK? Method Status   Protocol
FastEthernet0/0        192.168.1.1      YES manual up       up
Serial0/0/0            200.100.100.1    YES manual up       up
Serial0/0/1            unassigned       YES unset  down     down

Se0/0/0 still down? The serial link will only come up once R2 is also configured and its interface is brought up. Carry on with Part 3 — it will turn green once both ends are up.

Part 3 — Configure R2

Time: ~15 min • Click R2 → CLI.

3.1 Set hostname and configure LAN interface (Fa0/0)

Router> enable
Router# configure terminal
Router(config)# hostname R2

R2(config)# interface Fa0/0
R2(config-if)# ip address 192.168.0.1 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# exit

3.2 Configure the serial WAN interface (Se0/0/0) — DTE

R2 has the DTE end of the serial cable — no clock rate is needed here. R1 already provides the clock.

R2(config)# interface Se0/0/0
R2(config-if)# ip address 200.100.100.254 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# exit

R2(config)# end
R2# write memory

After this command, go back and check R1 — Se0/0/0 on both routers should now show up/up.

3.3 Verify R2 interfaces

R2# show ip interface brief

Expected output:

Interface              IP-Address         OK? Method Status   Protocol
FastEthernet0/0        192.168.0.1        YES manual up       up
Serial0/0/0            200.100.100.254    YES manual up       up

Part 4 — Configure Static Routes

Time: ~15 min

Right now each router knows only about its own directly connected networks. R1 knows 192.168.1.0/24 and 200.100.100.0/24, but has no idea how to reach 192.168.0.0/24 (Branch B) — and vice versa for R2. You need to tell each router about the remote network manually using a static route.

4.1 Add a static route on R1 to reach Branch B

R1 needs to know: “to reach 192.168.0.0/24, send packets to 200.100.100.254 (R2's serial interface).”

R1# configure terminal
R1(config)# ip route 192.168.0.0 255.255.255.0 200.100.100.254
R1(config)# end
R1# write memory

4.2 Add a static route on R2 to reach Branch A

R2 needs to know: “to reach 192.168.1.0/24, send packets to 200.100.100.1 (R1's serial interface).”

R2# configure terminal
R2(config)# ip route 192.168.1.0 255.255.255.0 200.100.100.1
R2(config)# end
R2# write memory

4.3 Verify the routing tables

R1# show ip route

Look for these route types:

C — Connected: networks R1 is directly attached to (LAN-A and the serial link).
S — Static: the route you just added to reach Branch B.

You should see a line like:

S    192.168.0.0/24 [1/0] via 200.100.100.254

Run the same command on R2 and confirm it has a static route to 192.168.1.0/24.

Reading the routing table. [1/0] means Administrative Distance = 1 (static) and metric = 0. Lower AD is more trusted. You will see AD values in the lecture slides for all route types.

Part 5 — Verify Branch-to-Branch Connectivity

Complete every test before saving your file. All tests should pass before you move to Session 2.

PC-A1 → ping 192.168.1.11 (PC-A2) — same LAN, should succeed ✅
PC-B1 → ping 192.168.0.11 (PC-B2) — same LAN, should succeed ✅
PC-A1 → ping 192.168.1.1 (R1 Fa0/0 gateway) — should succeed ✅
PC-B1 → ping 192.168.0.1 (R2 Fa0/0 gateway) — should succeed ✅
PC-A1 → ping 200.100.100.1 (R1 serial) — should succeed ✅
PC-A1 → ping 200.100.100.254 (R2 serial) — should succeed ✅
PC-A1 → ping 192.168.0.10 (PC-B1) — cross-router, should succeed ✅
PC-B1 → ping 192.168.1.10 (PC-A1) — cross-router, should succeed ✅
PC-A1 → tracert 192.168.0.10 — should show two hops: R1 then PC-B1 ✅
show ip route on R1 — shows S 192.168.0.0/24 and C connected routes ✅
show ip route on R2 — shows S 192.168.1.0/24 and C connected routes ✅

Save now. Use File → Save. You will extend this file in Session 2. This working copy is also your reset point for Part 9.

Session 2 — Extend to Three Routers (Internet)

Part 6 — Add R3 and the Internet Link

Time: ~25 min • Open your Session 1 .pkt file.

6.1 Add new devices

Add a 2811 Router to the canvas — rename it R3. This represents the ISP/edge router.
Add a 2960-24TT Switch — rename it SW-C. This is the “internet-side” LAN.
Add a Server — rename it Web-Server.

6.2 Add WIC-2T module to R3

Click R3 → Physical.
Power R3 OFF.
Drag a WIC-2T (or WIC-1T) module into an empty slot.
Power R3 back ON.

6.3 Connect new cables

Cable type	From device / port	To device / port
Serial DCE	R1 `Se0/0/1` — DCE end	R3 `Se0/0/0` — DTE end
Copper Straight-Through	R3 `Fa0/0`	SW-C — `Fa0/24`
Copper Straight-Through	Web-Server	SW-C — `Fa0/1`

Click R1’s Se0/0/1 first when placing the serial cable so R1 gets the DCE end and provides the clock signal.

6.4 Configure Web-Server static IP

Click Web-Server → Desktop → IP Configuration → Static:

IP Address: 10.0.0.10
Subnet Mask: 255.255.255.0
Default Gateway: 10.0.0.1

Part 7 — Configure R3 and Default Routes

Time: ~20 min

7.1 Configure R3 interfaces

Click R3 → CLI:

Router> enable
Router# configure terminal
Router(config)# hostname R3

R3(config)# interface Se0/0/0
R3(config-if)# ip address 172.16.0.2 255.255.255.252
R3(config-if)# no shutdown
R3(config-if)# exit

R3(config)# interface Fa0/0
R3(config-if)# ip address 10.0.0.1 255.255.255.0
R3(config-if)# no shutdown
R3(config-if)# exit

R3(config)# end
R3# write memory

Verify R3:

R3# show ip interface brief

Both Se0/0/0 and Fa0/0 should show up/up.

7.2 Add the missing R1 serial interface for the R1↔R3 link

R1 already exists but Se0/0/1 is unconfigured. Open R1 → CLI:

R1# configure terminal
R1(config)# interface Se0/0/1
R1(config-if)# ip address 172.16.0.1 255.255.255.252
R1(config-if)# clock rate 64000
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# end
R1# write memory

The R1↔R3 serial link should now be up/up on both ends.

7.3 Add static routes on R3 (return path to both branches)

R3 has no idea how to reach Branch A or Branch B. You need to add specific static routes pointing back through R1:

R3# configure terminal
R3(config)# ip route 192.168.1.0 255.255.255.0 172.16.0.1
R3(config)# ip route 192.168.0.0 255.255.255.0 172.16.0.1
R3(config)# end
R3# write memory

Why does R3 need return routes? Routing is always two-way. If PC-A1 pings the Web-Server, the ping request travels outward. But the reply from Web-Server travels back — R3 must know how to send that reply to 192.168.1.10. Without these routes R3 drops the reply and the ping times out.

7.4 Replace specific routes on R1 and R2 with default routes

Instead of adding a specific static route to every internet destination (impossible in practice), R1 and R2 use a default route — a catch-all that says “if you don’t know where else to send it, send it here.”

On R1 — add a default route to R3, and remove the old specific route to Branch B (R2 will now handle that via its own default route):

R1# configure terminal
R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.0.2
R1(config)# end
R1# write memory

Keep the Branch B route on R1. Do not remove ip route 192.168.0.0 255.255.255.0 200.100.100.254 from R1. R1 still needs that specific route to reach Branch B via R2. The default route only covers everything R1 doesn’t have a specific entry for (e.g., 10.0.0.0/24).

On R2 — add a default route pointing to R1. R2 will send all unknown traffic (including internet traffic) to R1, which will forward it onward to R3:

R2# configure terminal
R2(config)# ip route 0.0.0.0 0.0.0.0 200.100.100.1
R2(config)# end
R2# write memory

7.5 Verify the extended routing tables

R1# show ip route

You should now see three types of entries on R1:

C — Connected: 192.168.1.0/24, 200.100.100.0/24, 172.16.0.0/30
S — Static: 192.168.0.0/24 via 200.100.100.254
S* — Default static: 0.0.0.0/0 via 172.16.0.2

R2# show ip route

R2 should show:

C — Connected: 192.168.0.0/24, 200.100.100.0/24
S — Static: 192.168.1.0/24 via 200.100.100.1
S* — Default static: 0.0.0.0/0 via 200.100.100.1

S* means gateway of last resort. The asterisk marks the default route. Any packet that doesn’t match a more specific route gets sent here. This is exactly what your home router does — it has a default route pointing to your ISP.

Part 8 — Verify Full Three-Site Connectivity

Complete every test. Do not proceed to Part 9 until all pass.

PC-A1 → ping 192.168.0.10 (PC-B1) — branch to branch ✅
PC-B1 → ping 192.168.1.10 (PC-A1) — branch to branch ✅
PC-A1 → ping 10.0.0.10 (Web-Server) — branch to internet ✅
PC-B1 → ping 10.0.0.10 (Web-Server) — branch to internet ✅
PC-A1 → tracert 10.0.0.10 — should show hops: R1 → R3 → Web-Server ✅
PC-B1 → tracert 10.0.0.10 — should show hops: R2 → R1 → R3 → Web-Server ✅
show ip route on R1 — shows S* default route and S to Branch B ✅
show ip route on R2 — shows S* default route and S to Branch A ✅
show ip route on R3 — shows two S routes back to both branches ✅

Save your file now. Keep this working copy. You will use it as the reset point for Part 9 fault scenarios.

Session 3 — Break & Fix Fault Scenarios (Pairs)

Part 9 — Break & Fix Fault Scenarios

Mode: Pairs. Student A introduces a fault (following the steps below) while Student B looks away. Student B then diagnoses and fixes the network using show commands and the CLI reference above. Swap roles after each fault. Record everything in the Fault Worksheet.

Important. After fixing each fault, verify the affected ping test from Part 8 passes again before introducing the next fault. Reload from your saved file if things become confusing.

Fault 1

Wrong next-hop IP in a static route

How to introduce it

R1# configure terminal
R1(config)# no ip route 192.168.0.0 255.255.255.0 200.100.100.254
R1(config)# ip route 192.168.0.0 255.255.255.0 200.100.100.253
R1(config)# end

Symptom clue

PC-A1 can ping R1 and the serial link, but ping 192.168.0.10 (PC-B1) times out. PC-B1 is completely unreachable from Branch A. Branch B to internet still works.

Diagnostic commands

R1# show ip route
R1# show ip route static

The static route to 192.168.0.0 will show via 200.100.100.253 — an address that doesn’t exist on R2.

The fix

R1(config)# no ip route 192.168.0.0 255.255.255.0 200.100.100.253
R1(config)# ip route 192.168.0.0 255.255.255.0 200.100.100.254
R1(config)# end
R1# write memory

Fault 2

Missing return route on R3 (asymmetric routing)

How to introduce it

R3# configure terminal
R3(config)# no ip route 192.168.0.0 255.255.255.0 172.16.0.1
R3(config)# end

Symptom clue

PC-A1 can ping the Web-Server successfully (Branch A → internet works). But PC-B1 cannot ping the Web-Server at all — ping 10.0.0.10 times out. Branch B to Branch A still works fine.

This is a tricky one. The outbound path from Branch B works (packets reach R3). But R3 has no route back to 192.168.0.0/24, so the reply is dropped. This is called asymmetric or one-way routing and is one of the hardest faults to diagnose because the problem is in the return path, not the forward path.

Diagnostic commands

R3# show ip route

Notice that 192.168.0.0/24 is missing from R3’s routing table, while 192.168.1.0/24 is still present. The Web-Server and R3 have no way to send replies back to Branch B.

The fix

R3(config)# ip route 192.168.0.0 255.255.255.0 172.16.0.1
R3(config)# end
R3# write memory

Fault 3

Router LAN interface shut down

How to introduce it

R2# configure terminal
R2(config)# interface Fa0/0
R2(config-if)# shutdown
R2(config-if)# end

Symptom clue

PC-B1 and PC-B2 cannot ping anything — not even their own gateway. Branch A to internet continues to work normally. Pings from Branch A to 192.168.0.x all time out.

Diagnostic commands

R2# show ip interface brief

Look for Fa0/0 showing administratively down / down. The serial interface will still be up, which is a clue that the problem is on the LAN side, not the WAN.

The fix

R2(config)# interface Fa0/0
R2(config-if)# no shutdown
R2(config-if)# end
R2# write memory

Fault 4

Wrong subnet mask in a static route

How to introduce it

R2# configure terminal
R2(config)# no ip route 192.168.1.0 255.255.255.0 200.100.100.1
R2(config)# ip route 192.168.1.0 255.255.0.0 200.100.100.1
R2(config)# end

Symptom clue

PC-B1 can ping PC-A1 at 192.168.1.10 — but PC-B1 cannot reach anything in the 192.168.2.x or 192.168.3.x range (which don’t exist here, but the wrong mask makes R2 think it owns a much larger network). More usefully: tracert 192.168.1.10 from PC-B1 shows the route is correct, but the routing table looks suspicious.

Why does the ping still work? The wrong mask (255.255.0.0 = /16) is a superset of the correct mask (/24), so 192.168.1.x is still covered. The bug would cause routing failures to other 192.168.x.x subnets if they existed. It’s a time-bomb misconfiguration.

Diagnostic commands

R2# show ip route static
R2# show running-config | include ip route

The static route will show 192.168.1.0/16 instead of 192.168.1.0/24.

The fix

R2(config)# no ip route 192.168.1.0 255.255.0.0 200.100.100.1
R2(config)# ip route 192.168.1.0 255.255.255.0 200.100.100.1
R2(config)# end
R2# write memory

Fault Worksheet

Complete this table as you work through Part 9. Write in your own words — a sentence or two per cell is enough.

#	Symptom you observed	Commands you ran	Root cause	Fix you applied
1
2
3
4

Reflection Questions

In Fault 1, the wrong next-hop IP was only one digit off (.253 instead of .254). Explain why this completely breaks routing to Branch B even though the rest of R1’s configuration is correct.
Fault 2 demonstrated asymmetric routing. In your own words: why could PC-A1 ping the Web-Server successfully, but PC-B1 could not — even though both are on the same physical path to R3?
You used show ip route heavily in this lab. What is the difference between a C (connected) route and an S (static) route? Which one can the router configure itself?
In Session 2 you added a default route on R1 and R2 instead of specific routes to 10.0.0.0/24. What would happen if the internet had 1 million different networks — why is a default route essential in real-world routing?
If a student said “pings work in one direction but not the other,” which router would you check first and what command would you run? (Hint: think about which device handles the return path.)