What are the most important Cisco IOS commands to memorize for CCNA?
The most critical CCNA IOS commands are: show ip route (routing table), show ip interface brief (interface status), show running-config (current configuration), show version (IOS version and hardware), ping and traceroute (connectivity testing), interface configuration commands (ip address, no shutdown), and OSPF configuration commands (router ospf, network, area). These commands appear across multiple exam questions and simulation tasks.
The Cisco CCNA 200-301 exam includes simulation questions (also called lab questions) that require candidates to enter IOS commands in a simulated terminal. These questions cannot be answered by selecting an option -- they require typing commands with correct syntax. Candidates who have not practiced IOS commands extensively will struggle with these questions even if they understand the underlying concepts.
This cheat sheet provides the essential IOS commands organized by functional area. It is designed to be reviewed regularly during the final weeks of CCNA preparation as a recall exercise and quick reference during Packet Tracer practice.
Exec Mode Command Reference
Show Commands (Most Frequently Tested)
| Command | Output | Use Case |
|---|---|---|
show ip route |
Full routing table | Verify routes, check routing protocol |
show ip interface brief |
Interface IP and status | Quick interface status check |
show interfaces |
Detailed interface stats | Troubleshooting errors, speed/duplex |
show running-config |
Current configuration | Verify all configuration |
show startup-config |
Saved configuration | Verify what survives reload |
show version |
IOS version, uptime, hardware | System identification |
show ip protocols |
Routing protocol configuration | Verify OSPF, EIGRP parameters |
show ip ospf neighbor |
OSPF adjacency table | Verify OSPF neighbor relationships |
show ip ospf database |
OSPF link state database | Advanced OSPF troubleshooting |
show vlan brief |
VLAN list with port assignments | Verify VLAN configuration |
show interfaces trunk |
Trunk port configuration | Verify trunking |
show spanning-tree |
STP topology | Verify STP root, port states |
show mac address-table |
MAC address table | Verify learned MAC addresses |
show cdp neighbors |
Directly connected Cisco devices | Network topology discovery |
show ip nat translations |
Active NAT translations | Verify NAT operation |
show access-lists |
ACL content and hit counts | Verify ACL rules |
Interface Configuration
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Router(config-if)# no shutdown
Router(config-if)# description WAN link to ISP
Router(config-if)# duplex full
Router(config-if)# speed 1000
Router(config-if)# exit
Key interface facts for the exam:
- Interfaces are administratively down by default on routers (must use
no shutdown) - Switches have interfaces up by default if connected
show ip interface briefshows "administratively down" when shutdown is applied
OSPF Configuration (Single Area)
Router(config)# router ospf 1
Router(config-router)# network 192.168.1.0 0.0.0.255 area 0
Router(config-router)# network 10.0.0.0 0.255.255.255 area 0
Router(config-router)# passive-interface GigabitEthernet0/1
Router(config-router)# router-id 1.1.1.1
OSPF key concepts:
- Wildcard mask is the inverse of subnet mask (255.255.255.0 mask = 0.0.0.255 wildcard)
- Area 0 is the backbone area; all other areas must connect to it
- passive-interface stops OSPF hellos on that interface (use for user-facing interfaces)
- router-id is highest loopback IP by default if not manually configured
OSPF neighbor states (in order): Down > Init > 2-Way > ExStart > Exchange > Loading > Full
"OSPF neighbor troubleshooting is a staple of CCNA simulation questions. If neighbors are stuck at 2-Way, check DR/BDR election. If stuck at ExStart, check MTU mismatch." -- CCNA exam preparation guidance
VLANs and Trunking
Creating VLANs
Switch(config)# vlan 10
Switch(config-vlan)# name Sales
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name Engineering
Assigning Access Ports
Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Configuring Trunk Ports
Switch(config)# interface GigabitEthernet0/24
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30
Router-on-a-Stick (Inter-VLAN Routing)
Router(config)# interface GigabitEthernet0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0
Access Control Lists
Standard ACL (matches source IP only)
Router(config)# access-list 10 permit 192.168.1.0 0.0.0.255
Router(config)# access-list 10 deny any
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip access-group 10 in
Extended ACL (matches source, destination, protocol, port)
Router(config)# ip access-list extended BLOCK_HTTP
Router(config-ext-nacl)# deny tcp any any eq 80
Router(config-ext-nacl)# permit ip any any
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip access-group BLOCK_HTTP in
ACL placement rule:
- Standard ACLs: place close to destination
- Extended ACLs: place close to source
DHCP Server Configuration
Router(config)# ip dhcp pool LAN_POOL
Router(dhcp-config)# network 192.168.1.0 255.255.255.0
Router(dhcp-config)# default-router 192.168.1.1
Router(dhcp-config)# dns-server 8.8.8.8
Router(dhcp-config)# lease 7
Router(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.10
NAT Configuration
Static NAT
Router(config)# ip nat inside source static 192.168.1.10 203.0.113.5
Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip nat inside
Router(config)# interface GigabitEthernet0/1
Router(config-if)# ip nat outside
PAT (NAT Overload)
Router(config)# access-list 1 permit 192.168.0.0 0.0.255.255
Router(config)# ip nat inside source list 1 interface GigabitEthernet0/1 overload
Troubleshooting Commands
| Command | What It Tests | Common Use |
|---|---|---|
ping 8.8.8.8 |
Layer 3 connectivity | Basic reachability |
ping 8.8.8.8 source gi0/0 |
Source-specific reachability | Routing asymmetry |
traceroute 8.8.8.8 |
Path and hop latency | Route tracing |
debug ip ospf events |
Real-time OSPF events | OSPF adjacency issues |
debug ip routing |
Route table changes | Routing convergence |
no debug all |
Stop all debugging | Required after debug commands |
"Always run 'no debug all' after any debug session. Debug output on a production router can cause the device to reload under high load. This appears as an exam scenario question." -- Cisco networking certification instructor
Subnetting Quick Reference
| Prefix | Subnet Mask | Hosts per Subnet |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
Formula: Hosts per subnet = 2^(32-prefix) - 2
Frequently Asked Questions
How many IOS commands are on the CCNA exam? The CCNA exam tests approximately 50-100 distinct commands across show commands, configuration commands, and troubleshooting commands. Simulation questions typically involve 5-15 commands per question. The commands listed in this cheat sheet cover the core set that appears most frequently.
Do I need to memorize the exact syntax for every command? Yes, for simulation questions. The simulated terminal does not allow tab completion or abbreviated commands the way real IOS does. You must type commands with correct syntax. However, show commands and interface configuration commands account for the majority of simulation question requirements.
What is the best way to practice IOS commands? Cisco Packet Tracer is the recommended free practice tool. Download it from Cisco NetAcad (free registration required) and configure topologies from the labs in your CCNA study guide. GNS3 with IOS images provides a more realistic environment but requires more setup.
References
- Odom, W. (2020). CCNA 200-301 Official Cert Guide, Volume 1. Cisco Press.
- Odom, W. (2020). CCNA 200-301 Official Cert Guide, Volume 2. Cisco Press.
- Cisco Systems. (2024). Cisco IOS Command Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/fundamentals/command/Cisco_IOS_Fundamentals_Command_Reference.html
- Cisco Systems. (2024). Cisco Packet Tracer. https://www.netacad.com/courses/packet-tracer
- Cisco Systems. (2024). CCNA 200-301 Exam Topics. https://learningnetwork.cisco.com/s/ccna-exam-topics
- Empson, S. (2020). CCNA 200-301 Portable Command Guide. Cisco Press.