show version – IOS Version, Config Register, Reload Reason & Upgrade Checks
1. Purpose of show version
show version is the single most informative diagnostic command on
any Cisco device. It provides a complete hardware and software snapshot —
everything needed to identify a device, verify its software state, check
resources for an upgrade, understand why it rebooted, and confirm licensing.
It runs from privileged exec mode (Router#) with no parameters
required.
| Use Case | What to Look For |
|---|---|
| Pre-upgrade check | IOS version, RAM size, Flash size, platform model — determine if the new image fits and is supported |
| Diagnose unexpected reboot | System uptime (how recently), last reload reason (power-on, crash, reload command) |
| Hardware inventory | Model number, Processor board ID (serial number), interface count — for asset management and Cisco TAC cases |
| Password recovery | Configuration register value — must be 0x2142 before reboot and returned to 0x2102 afterward |
| License / feature verification | License level (IP Base, IP Services, Security, Voice) — determines which features are available |
| Boot troubleshooting | System image file location (flash:), ROMMON version, boot variable |
Related pages:
Related pages: debug ip packet | show ip route | show running-config | show interfaces | show logging | show mac address-table | AAA & Password Recovery | OSPF Overview | IOS Upgrade Lab | Password Recovery Lab
2. Full Annotated show version Output — Cisco Router (IOS 15.x)
Router# show version
Cisco IOS Software, Version 15.7(3)M2, RELEASE SOFTWARE (fc1) <-- IOS version
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2018 by Cisco Systems, Inc.
Compiled Mon 26-Mar-18 15:15 by prod_rel_team
ROM: System Bootstrap, Version 15.7(3r)M2, RELEASE SOFTWARE (fc1) <-- ROMMON version
(bootstrap loader)
Router uptime is 1 week, 2 days, 14 hours, 7 minutes <-- time since last reboot
System returned to ROM by reload at 08:23:41 UTC Mon Mar 12 2018 <-- last reload reason
+ timestamp
System image file is "flash0:isr4300-universalk9.03.16.07.S.155-3.S7-ext.SPA.bin"
^ ^ ^ ^
| Platform | Feature set
Flash IOS XE version Version string
Last reload type: Normal Reload
Last reload reason: Reload command <-- administrator issued 'reload'
cisco ISR4331/K9 (1RU) processor with 1647578K/6147K bytes of memory.
^ ^ ^
Model DRAM Shared I/O memory
Processor board ID FGL213722RX <-- Serial number
3 Gigabit Ethernet interfaces <-- Interface inventory
2 Management Ethernet interfaces
32768K bytes of non-volatile configuration memory. <-- NVRAM size
4194304K bytes of physical memory. <-- Total RAM (4 GB)
3255295K bytes of flash memory at bootflash:. <-- Flash storage
License Information for Module:'esg'
Level: appxk9 <-- License level
Type: Smart License
Configuration register is 0x2102 <-- Boot config register
^^^^
Normal boot = load startup-config from NVRAM
3. Full Annotated show version Output — Cisco Switch (IOS 15.x)
Switch# show version
Cisco IOS Software, C2960X Software (C2960X-UNIVERSALK9-M), Version 15.2(7)E1
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2019 by Cisco Systems, Inc.
Compiled Thu 03-Oct-19 09:49 by prod_rel_team
ROM: Bootstrap program is C2960X boot loader
BOOTLDR: C2960X Boot Loader (C2960X-HBOOT-M) Version 15.2(7r)E1 <-- Boot loader version
Switch uptime is 34 weeks, 2 days, 5 hours, 17 minutes
System returned to ROM by power-on <-- Reload reason: power cycle
(not a manual reload)
System image file is "flash:c2960x-universalk9-mz.152-7.E1.bin"
Processor board ID FOC2141Y23P <-- Serial number
Last reload type: Normal Reload
cisco WS-C2960X-48FPS-L (APM86XXX) processor (revision V01) <-- Model number
^^^^^^^
48 ports, PoE, L2 only
with 524288K bytes of memory.
2048K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address : A8:9D:21:FE:BC:00 <-- Switch base MAC
Motherboard assembly number : 73-16169-06
Power supply part number : 341-0543-01
Motherboard serial number : FOC21411YRL
Power supply serial number : POG21431VXT
Model revision number : V01
Motherboard revision number : A0
Model number : WS-C2960X-48FPS-L
System serial number : FOC2141Y23P <-- same as Processor board ID
48 FastEthernet interfaces <-- Interface summary
4 Gigabit Ethernet interfaces
2 Ten Gigabit Ethernet interfaces
Configuration register is 0x102 <-- Switch: register
^^^ (no leading 2 on some switch models)
4. Key Fields — Detailed Reference
| Field | What It Contains | Why It Matters |
|---|---|---|
| IOS Version | Version string: e.g., 15.7(3)M2, 15.2(7)E1, 16.9.4 | Identifies current software; determines feature availability; required before downloading a replacement image (must match platform and licence) |
| System Image File | Flash path and filename of loaded IOS binary | Confirms which image is running; the filename encodes platform, feature set, and version (see Section 5) |
| Device Model | Hardware platform identifier (e.g., ISR4331/K9, WS-C2960X-48FPS-L) | Required for Cisco TAC cases, licensing, and selecting the correct IOS image for upgrade |
| Processor Board ID | Device serial number (e.g., FGL213722RX, FOC2141Y23P) | Unique hardware identifier for asset management, SmartNet contracts, and Cisco support cases |
| System Uptime | Time since last reload: weeks, days, hours, minutes | Short uptime + unexpected reload reason = investigate power or stability issue. Long uptime = stable device |
| Last Reload Reason | Why the device rebooted: reload command, power-on, software crash, watchdog timeout, etc. | Distinguishes planned vs. unplanned restarts; crash-triggered reloads require log investigation |
| RAM (Memory) | DRAM in KB or MB; format varies by platform | New IOS images have minimum RAM requirements; confirm before downloading. IOS XE 16.x typically requires 4+ GB DRAM |
| Flash | Flash storage size (bootflash, flash0, etc.) | New IOS image must fit in available flash space alongside the current image (for dual-image rollback capability) |
| NVRAM | Non-volatile RAM size (stores startup-config) | Large configurations on complex devices require sufficient NVRAM. Rarely a constraint on modern hardware |
| Configuration Register | 16-bit hex value: 0x2102, 0x2142, 0x2100, 0x2101 | Controls boot behaviour; critical for password recovery and boot troubleshooting (see Section 6) |
| Interface Summary | Count and type of interfaces: "48 FastEthernet interfaces, 4 Gigabit Ethernet interfaces" | Verifies expected hardware is present and recognised; missing interfaces may indicate hardware failure or module not seated |
| License Level | Active feature licence: IP Base, IP Services, Security, appxk9, securityk9 | Determines available features; some protocols (OSPF, BGP, VPN) require a specific licence level |
| ROMMON Version | Bootstrap/boot loader version | Required for ROMMON-based recovery; occasionally updated separately from IOS |
5. Decoding the IOS Image Filename
The System Image File field contains the IOS image filename stored in flash. Each part of the filename is meaningful and identifies exactly what is loaded.
Classic IOS filename:
c2960x - universalk9 - mz . 152-7.E1 . bin
^^^^^^^ ^^^^^^^^^^^ ^^ ^^^^^^^^^ ^^^
Platform Feature set Format Version Extension
c2960x = Catalyst 2960X platform
universalk9= Universal image with all feature sets (K9 = encryption capable)
mz = compressed (z) image stored in RAM (m = runs from RAM)
152-7.E1 = IOS 15.2(7)E1 release
bin = binary executable
Common feature set codes:
ipbase = IP Base (basic routing/switching features)
ipservices = IP Services (adds OSPF, BGP, EIGRP, QoS, MPLS)
security = Security features (VPN, firewall)
universalk9= Universal (all features; licence controls what's active)
Common format codes:
mz = DRAM-based, compressed (most common for routers)
az = Flash-based, compressed (rare)
w = runs from ROM (very old)
IOS XE filename (ISR 4000, Catalyst 9000 series):
isr4300-universalk9.03.16.07.S.155-3.S7-ext.SPA.bin
^^^^^^^ ^^^^^^^^^^^ ^^^^^^^^ ^^^^^^^^^^^^^^^^^^
Platform Feature set IOS XE ver Classic IOS equiv version string
IOS XE examples:
16.9.4 = Fuji release (Catalyst 9300, 9400, 9500)
17.3.x = Amsterdam release (Catalyst 9000, ISR 1000/4000)
Note: "show version" on IOS XE shows BOTH the IOS XE version
AND the equivalent classic IOS version string
6. Configuration Register — All Values Explained
The configuration register is a 16-bit value stored in NVRAM that controls several aspects of device behaviour at boot time. It is displayed in hexadecimal. Understanding it is essential for password recovery and boot troubleshooting.
| Value | Meaning | When Used |
|---|---|---|
| 0x2102 | Default/Normal boot — load IOS from flash, load startup-config from NVRAM | Normal production operation; this is what every device should show under normal conditions |
| 0x2142 | Ignore startup-config at boot — device boots with empty configuration | Password recovery procedure — allows admin to enter device without the unknown enable password. Must be changed back to 0x2102 after recovery |
| 0x2100 | Boot to ROMMON (ROM Monitor) — stops at rommon> prompt | Low-level recovery; ROMMON-based file operations (xmodem transfer, manual boot); rarely set intentionally in production |
| 0x2101 | Boot from ROM (mini-IOS or bootstrap image) — bypasses flash IOS | Recovery when flash is corrupt or IOS image is missing; boots minimal-functionality IOS from ROM |
| 0x2104, 0x2106, etc. | Boot field bits 0–3 select which boot source; upper bits control console speed, break key, etc. | Specialised configurations; break key disable (bit 8); console speed override (bits 11–13) |
Bit Breakdown of 0x2102
0x2102 in binary: 0010 0001 0000 0010
Bit 0-3 (boot field) = 0010 = Boot from flash (normal)
Bit 6 = 0 = Ignore boot system commands: NO (use boot system commands)
Bit 8 = 0 = Break key during boot: ENABLED (0 = enabled; 1 = disabled)
Bit 13 = 1 = Boot from ROM on failures after boot attempts
0x2142 in binary: 0010 0001 0100 0010
^
Bit 6 = 1 = IGNORE startup-config on next boot
This single bit change is the entire password recovery trick!
Changing the Configuration Register
! Change config register from global config mode: Router(config)# config-register 0x2102 ! Set to normal boot Router(config)# config-register 0x2142 ! Set to ignore startup-config ! Change takes effect on NEXT reload (not immediately) Router(config)# end Router# reload ! Verify after reload: Router# show version ... Configuration register is 0x2102 <-- confirm it changed correctly
7. Reload Reasons — What Each Means
| Reload Reason Message | Meaning | Action Required |
|---|---|---|
System returned to ROM by reload command |
Administrator issued reload — planned reboot |
None — expected; note the timestamp to confirm it was during a maintenance window |
System returned to ROM by power-on |
Device was power-cycled or experienced a power failure | Investigate power supply and UPS; if unplanned, review power event logs and environmental conditions |
System returned to ROM by software-forced crash |
IOS encountered a critical error and forced a restart (similar to a kernel panic) | Check show logging
for the crash traceback;
collect crash dump (show stacks); open Cisco TAC
case; consider IOS upgrade |
System returned to ROM by watchdog timeout |
Hardware watchdog timer expired — CPU was too busy to reset the watchdog counter (CPU utilisation issue) | Check show processes cpu for runaway processes;
check for routing loops or broadcast storms causing high CPU |
System returned to ROM by bus error at PC |
Bus error — often indicates a software bug accessing invalid memory, or hardware memory fault | Open Cisco TAC case; decode the crash address; check for known bugs in Cisco bug toolkit; may require hardware replacement |
Reload requested by user from terminal |
Reload was initiated via a console, VTY, or scheduled reload command | None if planned; investigate if unexpected |
Multiple reboots:
show version only shows the most
recent reload reason. To see a history of reboots and error events, check
show logging. On IOS XE,
show platform software
process slot switch active r0 monitor provides additional crash
history.
8. RAM and Flash — Pre-Upgrade Resource Check
Before upgrading IOS, the two critical resources are DRAM (must meet the new image's minimum RAM requirement) and Flash (must have enough free space for the new image, ideally alongside the old one for rollback).
From show version output:
"with 1647578K/6147K bytes of memory"
^^^^^^^ ^^^^^
DRAM Shared I/O memory
1647578K = approximately 1.57 GB DRAM total
"3255295K bytes of flash memory at bootflash:"
^^^^^^^
3255295K ÷ 1024 ÷ 1024 ≈ 3.1 GB flash available
! Verify available flash space:
Router# dir flash:
Directory of flash:/
1 -rwx 499712 Oct 22 2018 10:32:12 +00:00 isr4300-universalk9.SPA.bin
2 drwx 4096 Oct 22 2018 10:30:00 +00:00 managed
3255295 bytes total (2755583 bytes free)
^^^^^^^
2.6 GB free -- plenty for another ~100 MB IOS image
! If flash is tight -- check specific free space:
Router# show flash: | include bytes
3255295 bytes total (2755583 bytes free)
! Cisco guidance for IOS upgrade: keep BOTH old and new image in flash
! until new image is confirmed stable; then delete old image
! Old image serves as rollback if new image has issues
IOS Image Size vs. Memory Requirements
| Platform | Typical Image Size | Min DRAM (IOS 15.x) | Min Flash |
|---|---|---|---|
| Catalyst 2960X | ~25 MB | 128 MB | 64 MB |
| Catalyst 3850 / IOS XE | ~200 MB | 4 GB | 4 GB |
| ISR 4000 / IOS XE | ~300 MB (package) | 4 GB | 8 GB |
| Catalyst 9300 / IOS XE | ~600 MB–1 GB | 4 GB | 16 GB |
9. Password Recovery Using Configuration Register
When an enable password or enable secret password is forgotten, the configuration register is used to bypass the startup configuration and regain access to the device. The key is that by setting the register to 0x2142, the router boots with a blank configuration — no password protection.
Password recovery procedure (Cisco router):
Step 1: Power cycle the router. Within 60 seconds of boot,
send a BREAK signal on the console to enter ROMMON mode.
(Ctrl+Break in PuTTY, or Break key on terminal)
Prompt: rommon 1 >
Step 2: Change config register to ignore startup-config:
rommon 1 > confreg 0x2142
rommon 2 > reset
(Router reboots and comes up with blank config)
Step 3: When prompted to enter setup mode, type No.
Router boots to: Router>
Type: enable
(No password required -- blank config)
Step 4: Restore configuration from startup-config:
Router# copy startup-config running-config
(This loads the full original config including the old password)
Step 5: Change the enable secret to the new password:
Router(config)# enable secret NewPassword123
Step 6: CRITICAL -- Restore normal config register:
Router(config)# config-register 0x2102
Router(config)# end
Router# copy running-config startup-config
Router# reload
(Device reboots with new password and normal boot behaviour)
Step 7: Verify:
Router# show version
...
Configuration register is 0x2102 <-- confirms normal boot restored
If you forget Step 6 (restoring 0x2102), the device will boot
with a blank configuration again on the next reload — losing all the
configuration changes made during recovery. Always verify the register
value with
show version before the final reload.
10. IOS vs IOS XE — Version Differences in show version
Cisco has been transitioning from classic IOS to IOS XE on newer
platforms. The show version output differs between them.
| Feature | Classic IOS (15.x) | IOS XE (16.x / 17.x) |
|---|---|---|
| Typical platforms | Catalyst 2960, 3750, 3560, ISR 2800/2900/3900 | Catalyst 9000 series, ISR 1000/4000/4400, ASR 1000 |
| Version format | 15.2(7)E1, 15.7(3)M2 | 16.9.4, 17.3.1 (also shows equivalent 15.x string) |
| IOS image | Single monolithic binary (.bin file) | Package-based (bundle or install mode; .bin or sub-packages) |
| Memory displayed | "with 524288K bytes of memory" | Shows both DRAM and package install info separately |
| Licence info | Legacy licence levels (IP Base, IP Services) | Cisco Smart Licensing — separate portal; show version shows feature level |
| show version appearance | Shorter output | More verbose; may show package install mode status |
IOS XE show version sample (Catalyst 9300):
Cisco IOS XE Software, Version 16.12.04 <-- IOS XE version
Cisco IOS Software [Gibraltar], Catalyst L3 Switch Software
(CAT9K_IOSXE), Version 16.12.4, RELEASE SOFTWARE (fc5)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2020 by Cisco Systems, Inc.
cisco C9300-48P (X86) processor with 1414933K/6147K bytes of memory.
Processor board ID FCW2312G07E
2048K bytes of non-volatile configuration memory.
8388608K bytes of physical memory.
1638400K bytes of Crashinfo at crashinfo:.
11264000K bytes of Flash at flash:. <-- 11 GB flash
0K bytes of WebUI ODM Files at webui:.
License Information for Module:'chassis 1'
License Level: network-essentials
License Type: Permanent Right-To-Use
Next reload license Level: network-essentials
The current throughput level is 250000 kbps
Configuration register is 0x102
11. Platform Differences — Router, Switch, ASA
| Device Type | Unique Fields in show version | Notable Differences |
|---|---|---|
| Router (ISR) | Serial/WAN interfaces, configuration register, processor type, ROMMON version, boot variable | Config register is critical; reload reasons include bus errors and watchdog timeouts from complex routing processes; licence determines routing feature set (e.g., MPLS, VPN) |
| Switch (Catalyst) | Ethernet interface summary (e.g., "48 FastEthernet, 4 Gigabit"), PoE status, base MAC address, switch serial number | Config register on some models uses 0x102 (without leading 2); licence determines if L3 routing is available; power supply info included on PoE models |
| Firewall (ASA) | Security context mode (single/multi), failover status, VPN session capacity, firewall mode (routed/transparent) | ASA uses different OS (ASA OS, not IOS); "show version" shows maximum VPN sessions, context count, failover configuration; licensing is per-context and per-feature |
| IOS XE (Cat 9000) | Both IOS XE version AND equivalent IOS version string, package install mode, Smart Licence status, throughput level | Much larger flash and RAM than classic IOS devices; install mode vs bundle mode affects upgrade procedure; Smart Licensing requires cloud or on-prem licence server |
12. Troubleshooting with show version — Practical Scenarios
| Scenario | What show version Reveals | Next Steps |
|---|---|---|
| Router rebooted unexpectedly overnight | Uptime = 4 hours (confirms recent reboot); Last reload reason = "software-forced crash" | show logging for crash traceback; show
stacks for process that crashed; open TAC case with
output; consider IOS upgrade |
| IOS upgrade fails — "not enough flash space" | Flash size = 64 MB; new image = 45 MB; old image = 35 MB (80 MB needed, only 64 MB total) | Delete old image ONLY after confirming new image boots correctly;
delete flash:old-image.bin; verify with
dir flash: |
| Feature not available after licence shows correct level | Licence level = "ipbase" but OSPF configuration fails | Upgrade licence to "ipservices" for OSPF/BGP/MPLS on classic IOS; on IOS XE, activate the appropriate Smart Licence entitlement |
| Device boots with no configuration | Config register = 0x2142 (startup-config being ignored) | Change to 0x2102: config-register 0x2102; save
the configuration; reload — or if this is intentional password
recovery, follow Section 9 procedure |
| Device stuck at rommon> prompt | Config register = 0x2100 (boot to ROMMON) | From ROMMON: confreg 0x2102 then reset;
or use ROMMON to manually boot:
boot flash:image.bin |
13. Related Diagnostic Commands
! ── After show version reveals crash reload reason ────────────────────────── Router# show logging ! Syslog buffer -- look for crash messages Router# show stacks ! Process stack at time of crash Router# show processes cpu sorted ! CPU utilisation by process Router# show processes memory sorted ! Memory utilisation ! ── Storage and image verification ────────────────────────────────────────── Router# dir flash: ! List flash contents and free space Router# dir bootflash: ! IOS XE platforms use bootflash: Router# verify /md5 flash:image.bin ! Verify image integrity (MD5 hash) Router# show flash: ! Summary of flash with filesystem info ! ── Boot configuration ─────────────────────────────────────────────────────── Router# show boot ! Boot variable and config register Router# show startup-config ! View startup-config in NVRAM ! ── Licensing ──────────────────────────────────────────────────────────────── Router# show license ! Licence status (classic IOS) Router# show license summary ! IOS XE Smart Licensing summary Router# show license udi ! Unique Device Identifier for Smart Licensing ! ── Hardware inventory ──────────────────────────────────────────────────────── Router# show inventory ! Detailed hardware inventory with S/N for all modules Router# show hardware ! Extended hardware details
14. Key Points & Exam Tips
show versionruns from privileged exec mode (#) — no configuration needed, no parameters required.- Configuration register 0x2102 = normal boot (load startup-config from NVRAM). 0x2142 = ignore startup-config (password recovery). 0x2100 = boot to ROMMON. These three values are critical exam knowledge.
- The config register change (
config-register 0x2142) takes effect at the next reload — not immediately. Always verify withshow versionafter reload to confirm it changed. - Reload reason interpretation: "power-on" = power failure or cycle; "reload command" = planned admin reload; "software crash" or "software-forced crash" = IOS bug → check logs and open TAC case.
- RAM check: look for "with XXXXXXK/XXXXK bytes of memory" — first
number is DRAM. Flash check: look for "XXXXXXK bytes of flash" or
run
dir flash:for precise free space. - The System Image File shows the running IOS filename including platform code, feature set, and version — all encoded in the filename.
- Processor Board ID = device serial number — needed for TAC cases and SmartNet support contract registration.
- IOS XE devices show both the IOS XE version (16.x, 17.x) and the
equivalent classic IOS version string in
show version. - Short uptime with unexpected reload reason = immediate investigation required. Long uptime (months/years) = stable production device.
- After password recovery, always restore 0x2102 with
config-register 0x2102before the final reload or the device boots with empty config again.
Related pages: debug ip packet | show ip route | show running-config | show interfaces | show logging | show mac address-table | AAA & Password Recovery | OSPF Overview | IOS Upgrade Lab | Password Recovery Lab