What is Cisco IOS? Features, Uses, and How to Access It

Cisco IOS (Internetwork Operating System) is the operating system running on most Cisco routers and switches. It provides the command-line interface, the routing and switching feature set, and the configuration model that has become the de facto standard for network CLIs, enough so that many other vendors offer an “IOS-like” mode.

Command Modes

The mode you are in determines which commands exist, and the prompt tells you which one that is.

exit moves back one level. end or Ctrl+Z jumps straight to privileged EXEC from anywhere.

Accessing It

Console, the out-of-band way in

A physical connection that works even when the network does not. Use it for initial setup, password recovery and any change that might cost you network access.

Remembered as 9600 8-N-1. Modern devices use USB or micro-USB console ports; older ones use an RJ45 console port with a rollover cable, the connector is 8P8C like Ethernet but carries RS-232 and is not electrically compatible with a network port.

SSH, the correct remote method

Router(config)# hostname R1
R1(config)# ip domain-name example.com
R1(config)# crypto key generate rsa modulus 2048
R1(config)# username admin privilege 15 secret StrongPassword
R1(config)# ip ssh version 2

R1(config)# line vty 0 15
R1(config-line)# transport input ssh
R1(config-line)# login local
R1(config-line)# exec-timeout 10 0

Three details matter here. The hostname and domain name must be set before generating keys, or the command fails. Use a 2048-bit modulus at minimum. And transport input ssh disables Telnet, which sends passwords in clear text and should be off everywhere.

Use secret rather than password: secret stores a proper hash, while password with service password-encryption uses a trivially reversible Type 7 obfuscation that is not encryption at all.

R1(config)# enable secret StrongEnablePassword
R1(config)# service password-encryption

Commands Worth Knowing

! What is this device?
show version                    ! model, IOS version, uptime, config register
show inventory                  ! hardware and serial numbers
show running-config             ! current configuration

! Interfaces
show ip interface brief         ! the fastest overview of every interface
show interfaces GigabitEthernet0/1
show interfaces status          ! switches, port status, VLAN, speed, duplex

! Layer 2
show vlan brief
show mac address-table
show spanning-tree
show cdp neighbors detail       ! what is plugged into what

! Layer 3
show ip route
show ip protocols
show ip arp

! Health
show processes cpu sorted
show memory statistics
show logging

Output filters, the ones that save the most time

show running-config | include ospf          ! matching lines only
show running-config | section interface     ! whole config sections
show running-config | begin line vty        ! from here onward
show interfaces | exclude 0 packets         ! hide matching lines
show ip route | count ^O                    ! count OSPF routes

| section is particularly useful, it returns the entire block, not just the matching line.

CLI Shortcuts

Two quality-of-life settings worth applying immediately on any device you work on:

R1(config)# no ip domain-lookup          ! stop typos being treated as hostnames
R1(config)# line console 0
R1(config-line)# logging synchronous     ! stop log messages interrupting your typing

Without no ip domain-lookup, mistyping a command makes the device try to resolve it as a hostname and hang for thirty seconds. It is the single most irritating default in IOS.

The Boot Process

1. POST, hardware self-test, from ROM.
2. Bootstrap loads from ROM.
3. The configuration register (normally 0x2102) determines where to look for the IOS image.
4. The IOS image loads from flash into RAM.
5. The startup config loads from NVRAM into RAM, becoming the running config.
6. If no startup config exists, the device offers initial setup mode.

R1# show version | include register
R1# show boot
R1(config)# boot system flash:c2900-universalk9-mz.SPA.157-3.M4.bin

Setting the register to 0x2142 skips step 5, which is how password recovery works.

IOS vs IOS-XE vs NX-OS

The practical significance of IOS-XE is fault isolation and automation. A crashed process can restart without taking the device down, and the platform supports model-driven configuration through NETCONF and RESTCONF, which is what network automation tooling targets.

Frequently Asked Questions

What are the console settings for a Cisco device?

9600 baud, 8 data bits, no parity, 1 stop bit, no flow control, “9600 8-N-1”.

What is the difference between enable password and enable secret?

enable secret stores a proper hash and takes precedence. enable password is stored in clear text, or with Type 7 obfuscation that is reversible in seconds. Always use secret.

How do I recover a lost password?

Console in, reboot, break into ROMMON, set the configuration register to 0x2142 so the startup config is skipped, boot, copy the startup config into running config, change the password, set the register back to 0x2102, and save.

Why does a mistyped command hang for 30 seconds?

IOS treats an unrecognised command as a hostname and attempts to resolve it via DNS. no ip domain-lookup stops this, and it should be one of the first commands on any lab device.

Is IOS-XE just a newer IOS?

Architecturally it is quite different, a Linux kernel running IOS as one process among several, which enables process restart, containers and model-driven programmability. The CLI is deliberately similar so existing knowledge transfers.

Where is the configuration stored?

The running config in RAM (lost on reboot) and the startup config in NVRAM (persistent). See running vs startup configuration.