Part 4 – Configure the Router-to-Router Connection
Part 4 – Configure the Router-to-Router Connection
Objective
In this lesson, you'll configure the Ethernet interfaces that connect Router A and Router B.
By the end of this lesson:
✅ Router A and Router B will have IP addresses on the link between them.
✅ The Ethernet interfaces will be enabled.
✅ The routers will be able to communicate directly.
Note: We are not adding static routes yet. We are only configuring the physical connection.
Step 1: Why Do We Need Another IP Address?
What is happening?
Router A is already connected to Network A.
Router B is already connected to Network B.
Now they need their own network to communicate with each other.
Think of it like this:
Network A
192.168.1.0/24
│
│
Router A
│
10.0.0.1/30
══════════════════
10.0.0.2/30
│
Router B
│
192.168.2.0/24
Network B
Why don't we use 192.168.1.x or 192.168.2.x?
Because those networks are already being used by the PCs.
The connection between the two routers should be its own separate network.
🌍 Real-Life Example
Imagine two office buildings.
Each building has its own internal roads.
To travel between the buildings, you build a private road.
That private road is separate from the roads inside either building.
The 10.0.0.0/30 network is that private road.
Step 2: Configure Router A's Second Interface
Where to do it
Open Router A.
Go to the CLI.
Enter Global Configuration Mode if you're not already there.
Type:
configure terminal
What to do
Enter the interface connected to Router B.
interface gigabitethernet0/1
What is GigabitEthernet0/1?
It is the second Ethernet port on Router A.
GigabitEthernet0/0 connects to the switch.
GigabitEthernet0/1 connects to the other router.
Assign the IP Address
Type:
ip address 10.0.0.1 255.255.255.252
Why are we using 255.255.255.252?
A /30 subnet provides exactly two usable IP addresses, which is perfect for a point-to-point connection between two routers.
Those two usable addresses are:
Router A → 10.0.0.1
Router B → 10.0.0.2
🌍 Real-Life Example
Imagine a bridge connecting only two islands.
Only two entrances are needed—one at each end.
A /30 network works the same way: it is designed for just two devices.
Step 3: Enable the Interface
Type:
no shutdown
What does it do?
It activates the Ethernet port.
Without this command, the interface remains disabled.
What should you see?
A message indicating the interface changed state to up.
Step 4: Configure Router B
Where to do it
Click Router B.
Open the CLI.
Enter Global Configuration Mode.
Configure the Interface
interface gigabitethernet0/1
Assign the IP address:
ip address 10.0.0.2 255.255.255.252
Enable the interface:
no shutdown
Why is Router B using 10.0.0.2?
Because every device on the same network must have a unique IP address.
Router A already has 10.0.0.1, so Router B uses 10.0.0.2.
🌍 Real-Life Example
Think of two friends talking on the phone.
Each person has a different phone number.
If both had the same number, the call couldn't work properly.
The routers also need different IP addresses to communicate.
Step 5: Verify the Interfaces
Where to do it
On both routers, type:
show ip interface brief
What does this command do?
It displays:
Interface names
Assigned IP addresses
Interface status
Protocol status
What should you see?
Something similar to:
GigabitEthernet0/0 192.168.1.1 up up
GigabitEthernet0/1 10.0.0.1 up up
On Router B:
GigabitEthernet0/0 192.168.2.1 up up
GigabitEthernet0/1 10.0.0.2 up up
What does up/up mean?
First "up" → The physical cable connection is working.
Second "up" → The communication protocol is working correctly.
If you see:
administratively down
It usually means you forgot to type:
no shutdown
Step 6: Test the Connection Between the Routers
Where to do it
On Router A, type:
ping 10.0.0.2
What is ping?
ping is a command that sends test packets to another device to check if it is reachable.
Why do we use it?
Before configuring routing, we must confirm that the two routers can communicate directly.
🌍 Real-Life Example
Imagine calling a coworker before starting an online meeting.
If they answer, you know the connection is working.
A ping does the same thing for network devices.
What should you see?
Replies similar to:
!!!!!
Each ! means one successful reply.
If you see:
.....
The routers are not communicating, so check:
Cable type
Correct interfaces
IP addresses
no shutdown
Part 4 Summary
You have successfully:
✅ Configured the Ethernet link between Router A and Router B.
✅ Assigned IP addresses to both router interfaces.
✅ Enabled the interfaces.
✅ Verified they are up/up.
✅ Tested connectivity using
ping.
Next Lesson: Part 5 – Configure Static Routing
In the next part, you'll learn:
What a routing table is.
Why routers need routes.
How to configure static routes.
How a packet travels from PC1 in Network A to PC6 in Network B through both routers.
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