Router & Switch Configuration

Overview

Initial router and switch setup follows a predictable progression: console access, hostname, management IP, VLANs, and static routes before dynamic routing. This guide walks that path with verification commands at each step.

Initial device setup — the standard progression

Whether you are configuring a router or switch, the first session follows the same pattern Jeremy's IT Lab and Panagiss both teach:

1. Console in — physical connection before any network exists
2. Enter privileged mode — enable
3. Enter global configuration — configure terminal
4. Set hostname — identify the device in prompts and logs
5. Configure management access — IP, gateway, SSH (security topic covers hardening)
6. Configure interfaces — IP addresses on routers; VLANs and access ports on switches
7. Verify — show commands before assuming anything works
8. Save — copy running-config startup-config or write memory

Hostname and basic identity

A descriptive hostname makes logs, CDP neighbors, and SSH sessions readable.

Naming convention example: NY-F1-SW1 (site-floor-role-number)

configure terminal
hostname SW1
!
interface FastEthernet0/1
description Link to R1

After setting the hostname, the prompt changes from Switch(config)# to SW1(config)#.

Router interface configuration

Routers need an IP address on every interface facing an IP subnet. Interfaces are administratively down by default on many platforms — you must enable them.

configure terminal
hostname R1
!
interface FastEthernet0/0
ip address 192.168.0.1 255.255.255.0
no shutdown
!
interface FastEthernet0/1
ip address 192.168.1.1 255.255.255.0
no shutdown
!
end

Each configured interface generates a connected route in the routing table automatically.

Switch management configuration

A Layer 2 switch forwards based on MAC addresses, but you configure a management IP on an SVI (Switched Virtual Interface) — typically VLAN 1 by default — so you can SSH to the switch from other subnets.

configure terminal
hostname SW1
!
interface vlan 1
ip address 192.168.0.10 255.255.255.0
no shutdown
!
ip default-gateway 192.168.0.1
!
end

The ip default-gateway command is required on L2 switches so management traffic to remote subnets reaches the router. Without it, you can only SSH to the switch from the same subnet.

Speed and duplex

Interfaces default to auto negotiation. Best practice on links between infrastructure devices: manually set matching speed and duplex on both sides.

interface FastEthernet0/1
duplex full
speed 100

VLAN creation and access ports

VLANs segment broadcast domains. Each access port belongs to exactly one VLAN.

configure terminal
vlan 10
name SALES
!
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
!
end

Verification:

show vlan brief
show interfaces switchport
show running-config interface GigabitEthernet0/1

Router-on-a-stick — inter-VLAN routing

When a single router connects to a switch via one physical link and routes between multiple VLANs, you use subinterfaces with 802.1Q tags. This is the classic CCNA inter-VLAN routing pattern.

Topology: SW1 trunk port → R1 Fa0/0. VLAN 10 (Sales) and VLAN 20 (Engineering) each get a subinterface.

configure terminal
hostname R1
!
interface FastEthernet0/0
no shutdown
!
interface FastEthernet0/0.10
encapsulation dot1Q 10
ip address 10.10.10.1 255.255.255.0
!
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 10.10.20.1 255.255.255.0
!
end

Switch side (trunk to router):

interface GigabitEthernet0/1
switchport mode trunk
switchport trunk allowed vlan 10,20

How it works:

1. PC in VLAN 10 (10.10.10.50) sends packet to PC in VLAN 20 (10.10.20.50)
2. Switch tags the frame with VLAN 10 on the trunk
3. R1 subinterface Fa0/0.10 receives, routes to Fa0/0.20
4. R1 sends frame back on trunk tagged VLAN 20
5. Switch delivers to VLAN 20 access port

Static routing basics

After interfaces are up, routers in a multi-router topology need routes to remote subnets.

! Specific network via next-hop
ip route 10.0.1.0 255.255.255.0 10.0.0.1
! Default route (gateway of last resort)
ip route 0.0.0.0 0.0.0.0 203.0.113.2

Essential verification commands

Build the habit of verifying after every configuration block.

show running-config
show startup-config
show ip interface brief
show interfaces status
show ip route
show vlan brief
show interfaces trunk
show mac address-table
show cdp neighbors detail
show version

Reading show ip interface brief

An interface showing up/down usually means Layer 1 is fine but Layer 2 failed (cable, mismatch, no neighbor).

CDP and LLDP — mapping connections

CDP (Cisco Discovery Protocol) shares device info at Layer 2 — enabled by default on Cisco gear.

show cdp neighbors
show cdp neighbors detail

LLDP (Link Layer Discovery Protocol) is the open-standard equivalent. May need lldp run in global config on some platforms.

Configuration management

! Save running config to NVRAM
copy running-config startup-config
write memory
! Factory reset (erases startup-config)
write erase
reload
! View interface-specific config
show running-config interface vlan 1

Cisco device memory (exam awareness)

Troubleshooting configuration — bottom-up

Panagiss maps troubleshooting to layers — apply this after every config change:

1. Layer 1 — show ip interface brief, show interfaces
2. Layer 2 — show arp, show mac address-table, show vlan brief
3. Layer 3 — show ip route, ping, traceroute
4. Layer 4 — telnet / TCP connection test to specific port

Complete lab walkthrough

Goal: SW1 with VLAN 10 (Sales) and VLAN 20 (Engineering). R1 routes between them via router-on-a-stick.

enable
configure terminal
hostname SW1
!
vlan 10
name SALES
vlan 20
name ENGINEERING
!
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
!
interface GigabitEthernet0/2
switchport mode access
switchport access vlan 20
!
interface GigabitEthernet0/24
switchport mode trunk
switchport trunk allowed vlan 10,20
!
interface vlan 1
ip address 192.168.0.10 255.255.255.0
no shutdown
!
ip default-gateway 192.168.0.1
!
end
write memory

enable
configure terminal
hostname R1
!
interface FastEthernet0/0
no shutdown
!
interface FastEthernet0/0.10
encapsulation dot1Q 10
ip address 10.10.10.1 255.255.255.0
!
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 10.10.20.1 255.255.255.0
!
end
write memory

Verify from a PC in VLAN 10: ping 10.10.20.1 (gateway on other VLAN) should succeed if routing is correct.

Exam checklist

• Configure hostname, interface IP, and no shutdown on a router
• Set management IP and default gateway on a switch
• Create a VLAN and assign an access port
• Configure router-on-a-stick with dot1Q subinterfaces
• Configure a switch trunk to a router
• Run verification commands and interpret show ip interface brief
• Save configuration with copy running-config startup-config