🌐 Phase 3 · Network Layer & Routing 🟡 Intermediate MODULE 10

Basic Router Configuration

⏱️ 3 hours

📖 5 sections

🧩 10 Questions

🏗️ Phase 3 · Module 10

CCNA Module 10 of 1759%

🎯 What you'll learn: Initial router configuration steps, configuring router interfaces (IPv4 and IPv6), verifying interface status with show commands, configuring the default gateway on hosts and switches, and connecting a router to a LAN.

Section 1

Initial Router Configuration

A Cisco router, fresh out of the box or reset to factory defaults, has no configuration at all. Bringing it into a working network follows the same basic sequence every time — set a hostname, secure access, configure interfaces, and save the configuration.

Hostname: uniquely identifies the device on the network and in your documentation. Always set this first.
Secure access: set passwords for privileged EXEC mode, console line, and VTY (remote/Telnet/SSH) lines.
Banner: a legal warning message displayed before login (Message of the Day).
Interfaces: assign IP addresses and enable each interface that will carry traffic.
Save: copy the running configuration to NVRAM so it survives a reboot.

Initial Router Configuration — Full Sequence

Cisco IOS

! Enter global configuration mode
Router> enable
Router# configure terminal

! Set the hostname
Router(config)# hostname R1

! Secure privileged EXEC mode
R1(config)# enable secret Cisco12345

! Secure the console line
R1(config)# line console 0
R1(config-line)# password ConsolePass1
R1(config-line)# login
R1(config-line)# exit

! Secure VTY lines (remote access — Telnet/SSH)
R1(config)# line vty 0 4
R1(config-line)# password VtyPass1
R1(config-line)# login
R1(config-line)# exit

! Encrypt all plaintext passwords in the running config
R1(config)# service password-encryption

! Add a Message of the Day banner
R1(config)# banner motd #Authorized access only#

! Save the configuration to NVRAM
R1(config)# end
R1# copy running-config startup-config

✨

enable secret vs enable password

enable secret stores an MD5-hashed password and always takes priority if both are configured. enable password stores plaintext (unless service password-encryption is applied, which only weakly obfuscates it). Always use enable secret — never rely on enable password alone in a production network.

Section 2

Configure Router Interfaces

A router does nothing useful until its interfaces are configured and enabled. Each routed interface needs an IP address and subnet mask, and must be turned on with no shutdown — interfaces are administratively down by default.

Always add a short description to each interface. This is not just good practice — on a real network with dozens of interfaces, descriptions are often the only thing standing between fast troubleshooting and hours of guesswork.

Configuring Router Interfaces — IPv4 and IPv6

Cisco IOS

! Configure GigabitEthernet0/0 — LAN-facing interface
R1(config)# interface GigabitEthernet0/0
R1(config-if)# description LAN - Connects to SW1
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# ipv6 address 2001:db8:1::1/64
R1(config-if)# no shutdown
R1(config-if)# exit

! Configure Serial0/0/0 — WAN-facing interface
R1(config)# interface Serial0/0/0
R1(config-if)# description WAN link to ISP
R1(config-if)# ip address 203.0.113.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# exit

⚠️

Forgetting "no shutdown" is the #1 router config mistake

Router interfaces start in an administratively down state. Even with a perfectly correct IP address and mask, an interface left shut down will never pass traffic. If a newly configured interface shows no connectivity, show ip interface brief should be the very first command you run.

Section 3

Verify Interface Configuration

After configuring interfaces, verification confirms everything is working as intended before moving on. Several show commands give different levels of detail — knowing which one to reach for saves time.

show ip interface brief

Quick Status Overview

One-line summary per interface: IP address, Status (up/down), Protocol (up/down). The fastest way to check everything at once.

show ip interface

Detailed Per-Interface Info

Full details for each interface — MTU, line protocol, ACLs applied, helper addresses, and more.

show running-config

Active Configuration

Shows the configuration currently running in RAM — including all interface settings, passwords (encrypted), and routing config.

show ipv6 interface brief

IPv6 Status Overview

Same idea as the IPv4 version, but lists link-local and global IPv6 addresses configured on each interface.

Interface Verification Output

Cisco IOS

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     192.168.1.1     YES manual up                    up
Serial0/0/0            203.0.113.1     YES manual up                    up
GigabitEthernet0/1     unassigned      YES unset  administratively down down

! "up / up" = working correctly
! "administratively down / down" = interface is shut down (no shutdown not applied)
! "down / down" = Layer 1 problem (cable, port, no signal)
! "up / down" = Layer 2 problem (encapsulation mismatch, keepalives)

💡

Reading Status / Protocol pairs

The first column (Status) reflects Layer 1 — is there a physical connection? The second column (Protocol) reflects Layer 2 — is the data link layer working? Both must show up for the interface to pass traffic. A mismatch between the two immediately tells you which OSI layer to investigate.

Section 4

Configure the Default Gateway

A default gateway is the address a host or switch sends traffic to when the destination is on a different network. Without one, devices can only communicate within their own local subnet.

On an end device (PC, laptop, phone), the default gateway is normally the LAN-facing IP address of the local router — set manually or via DHCP. On a Layer 2 switch, the default gateway matters for management traffic only (Telnet/SSH/SNMP to the switch itself) — switches don't route user traffic, so this setting doesn't affect how frames are forwarded between hosts.

Default Gateway — Switch and Windows Host

IOS + OS

! Configure default gateway on a Layer 2 switch (management only)
SW1(config)# ip default-gateway 192.168.1.1

! Configure SVI (Switch Virtual Interface) for management access
SW1(config)# interface vlan 1
SW1(config-if)# ip address 192.168.1.10 255.255.255.0
SW1(config-if)# no shutdown

! On a Windows host — set default gateway manually
C:\> netsh interface ip set address "Ethernet" static 192.168.1.50 255.255.255.0 192.168.1.1

! Verify on Windows
C:\> ipconfig
Default Gateway . . . . . . . . . : 192.168.1.1

🧭

If the gateway is wrong, only local traffic works

A host with the wrong (or missing) default gateway can still reach devices on its own subnet — but every attempt to reach a remote network silently fails, because the host has nowhere to send packets bound for an unfamiliar IP range. This is one of the most common "I can ping my neighbor but not the internet" problems.

Section 5

Connect a Router to a LAN

Putting it all together: a small topology with one router connecting a LAN switch to a WAN link demonstrates the full configuration workflow in one place.

🔌 Router-to-LAN Topology

PC1 (192.168.1.50/24)
Default Gateway: 192.168.1.1

↓

SW1 — Layer 2 Switch
(no IP routing, just forwards frames)

↓

R1 Gi0/0: 192.168.1.1/24 (LAN)
R1 S0/0/0: 203.0.113.1/30 (WAN)

↓

ISP Router / Internet

Step-by-step checklist for connecting a router to an existing LAN:

1. Cable the router's LAN interface to the switch using a straight-through Ethernet cable.
2. Configure the router's LAN interface with an IP address from the LAN subnet (e.g. 192.168.1.1/24) and no shutdown.
3. Configure each host on the LAN with an IP in the same subnet, and set its default gateway to the router's LAN interface address.
4. Verify Layer 1/2 with show ip interface brief — confirm up/up.
5. Verify end-to-end with ping from the host to the router, then from the host to a remote destination.
6. Save the configuration with copy running-config startup-config.

🔬

Challenge — Diagnose the Broken Link

Apply show commands to find a misconfiguration

Scenario: PC1 cannot reach the internet. You run show ip interface brief on R1 and see:

GigabitEthernet0/0 192.168.1.1 YES manual up up
Serial0/0/0 203.0.113.1 YES manual administratively down down

Task: Identify the problem and write the command(s) needed to fix it.

💡 Show hints & answer

"administratively down" always means the interface was never enabled, or was manually shut down
The fix: enter interface configuration mode for Serial0/0/0 and apply no shutdown
Re-verify with show ip interface brief — Status should change to "up"
If Protocol still shows down after that, check cabling and encapsulation settings on both ends of the WAN link

🧩 Knowledge Check

10 questions — Basic Router Configuration

1. Which command saves the running configuration to NVRAM?

2. By default, a newly configured router interface is:

3. Which command enables a previously shut down interface?

4. Which command gives the fastest one-line summary of every interface's IP and status?

5. An interface showing "administratively down / down" means:

6. Between enable secret and enable password, which takes priority if both are configured?

7. On a Layer 2 switch, the default gateway setting (ip default-gateway) is used for:

8. Which command encrypts plaintext passwords already stored in the configuration?

9. If Status shows "up" but Protocol shows "down" on an interface, the problem is most likely at:

10. What must a host have configured to reach networks beyond its own subnet?

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