13 – Configuring a Default Route (Gateway of Last Resort)

13 – Configuring a Default Route (Gateway of Last Resort)#

This tutorial is the thirteenth in our Cisco Packet Tracer series and focuses on default routing—also known as configuring a gateway of last resort. This type of route is essential in smaller or stub networks, where routers only have one exit path for unknown destinations.

We will build a small branch-office style topology and configure the branch router to forward all unknown traffic to a main router using a static default route.

Find the CISCO pkt files in the repo -

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Part 1 – Network Topology Overview#

This network includes:

  • Two routers (R0 = Branch, R1 = Main)

  • One switch per router (S0, S1)

  • Two PCs per switch (4 total PCs)

The branch site will have no knowledge of external networks, so it will use R1 as its gateway of last resort.

Figure

Part 2 – Device Placement and Cabling#

Step 2.1 – Add Devices to the Workspace#

From Network Devices and End Devices, place:

  • 2 Routers (Router-PT-Empty)

  • 2 Switches (2960)

  • 4 PCs

Label the devices:

  • Routers: R0 (Branch), R1 (Main)

  • Switches: S0 (for R0), S1 (for R1)

  • PCs: PC0–PC3

Step 2.2 – Add Network Modules to Routers#

Add:

  • 1x PT-ROUTER-NM-1S

  • 1x PT-ROUTER-NM-1CFE

Turn off each router, insert modules, then power back on.

If you need help with this, refer to the steps in How to Customise the Router-PT in Packet Tracer

Figure

Step 2.3 – Cabling#

Use Copper Straight-Through for PC to switch and switch to router connections. Use Serial DCE for the R0 ↔ R1 link.

From

To

Port/Interface

PC0

S0

fa0/1

PC1

S0

fa0/2

S0

R0

fa0/24 → fa2/0

PC2

S1

fa0/1

PC3

S1

fa0/2

S1

R1

fa0/24 → fa2/0

R0

R1

se0/0 ↔ se0/0

Part 3 – IP Addressing Scheme#

Now we will assign IP addresses to all devices, ensuring they can communicate across the network.

Subnet Allocation#

For this tutorial, we will use the following subnets:

Subnet

Devices

Subnet Mask

192.168.10.0/24

PC0, PC1, R0

255.255.255.0

192.168.20.0/24

PC2, PC3, R1

255.255.255.0

10.0.0.0/30

R0 ↔ R1

255.0.0.0

Assign IPs to PCs#

Go to Desktop > IP Configuration on each PC:

PC

IP Address

Subnet Mask

Default Gateway

PC0

192.168.10.11

255.255.255.0

192.168.10.1

PC1

192.168.10.12

255.255.255.0

192.168.10.1

PC2

192.168.20.13

255.255.255.0

192.168.20.1

PC3

192.168.20.14

255.255.255.0

192.168.20.1

Figure

Figure

Part 4 – Router Configuration#

In this part, we will configure the routers to enable communication between the PCs and set up a default route on R0.

Note

To ensure all unknown traffic from the branch site is forwarded to the main router, we configure a default route on R0. This is done with the command ip route 0.0.0.0 0.0.0.0 10.0.0.2, which instructs R0 to send any traffic destined for networks it doesn’t know about to R1. On R1, we add a static route pointing back to the branch network, allowing two-way communication.

Step 4.1 – R0 (Branch Router)#

enable
configure terminal
hostname R0

interface fa1/0
ip address 192.168.10.1 255.255.255.0
no shutdown
exit

interface se0/0
ip address 10.0.0.1 255.0.0.0
clock rate 64000
no shutdown
exit

ip route 0.0.0.0 0.0.0.0 10.0.0.2
exit
write memory

This is the default route. It says: “Send all unknown traffic to 10.0.0.2 (R1).”

Figure

Step 4.2 – R1 (Main Router)#

enable
configure terminal
hostname R1

interface fa1/0
ip address 192.168.20.1 255.255.255.0
no shutdown
exit

interface se0/0
ip address 10.0.0.2 255.0.0.0
no shutdown
exit

ip route 192.168.10.0 255.255.255.0 10.0.0.1
exit
write memory

Figure

Part 5 – Verification and Testing#

Step 5.1 – Check Routing Tables#

On R0:

show ip route

You should see a route labeled with S* – this is the static default route.

Figure

On R1:

show ip route

You should see a static route to 192.168.10.0 via 10.0.0.1

Figure

Step 5.2 – Ping Tests#

From PC0, try:

ping 192.168.20.10
ping 192.168.20.13

Figure

From PC3, try:

ping 192.168.20.13
ping 192.168.10.11

Figure

Summary#

In this tutorial, you:

  • Created a simple two-router topology

  • Assigned IP addresses and connected all devices

  • Configured a default route on the branch router

  • Verified full connectivity using ping and show ip route

Default routes are vital in small or remote networks where only one path leads to the rest of the network or the internet.

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