Cisco "show version" Command Explained
show version — reports the device's software version, uptime, hardware model, memory and the configuration register — the identity card of the box. Runs in user or privileged EXEC mode.
Syntax and common variants
| Variant | Purpose |
|---|---|
show version | Full platform and software report |
show version | include uptime | Just the uptime line |
show version | include register | Just the config-register |
Reading the output
| Output / element | Meaning |
|---|---|
Cisco IOS Software… Version | Exact IOS release running |
uptime is… | Time since last reload (sudden reset = clue) |
System returned to ROM by | WHY it last restarted (power, reload, crash) |
Model / memory lines | Hardware platform and RAM/flash |
Configuration register is 0x2102 | Boot behaviour (0x2142 = ignore startup config!) |
When to use it
First command in audits and TAC cases: exact version (for bug/security matching), uptime (did it just crash?), and the config register — the famous 0x2142 explains "router booted with empty config" after password recovery. Browse more in the command reference or practise in the free labs.
Frequently asked questions
What does the configuration register do?
It controls boot behaviour — 0x2102 is normal; 0x2142 tells the device to ignore startup-config (used in password recovery, must be set back).
How can I tell why a device rebooted?
The 'System returned to ROM by' line — power-on, reload command, or crash indications.
Why does the IOS version matter?
Features, bugs and vulnerabilities are version-specific — upgrades and security checks start from this output.
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