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IOS images are named like snowflake IDs and mean something specific. Decoding them tells you the platform, feature set, and version. And upgrading one is a project with a rollback plan — not a one-command operation you improvise at 2 a.m.
Take `c2900-universalk9-mz.SPA.157-3.M7.bin`. Read left to right:
The `k9` suffix means the image supports cryptographic features. Without it, SSH, IPSec, and other crypto functions aren't available. Almost every modern deployment uses k9 images.
Note the free space — 62 MB available. Any IOS image you want to add has to fit in that. Modern images are 200+ MB, so on older gear you often have to delete the current image before copying the new one, which means you have no fallback if the copy fails. Plan accordingly.
! Configure which image to bootR1-HQ(config)# boot system flash:c2900-universalk9-mz.SPA.157-3.M7.bin! Verify boot statementsR1-HQ# show bootvarBOOT variable = flash:c2900-universalk9-mz.SPA.157-3.M7.bin,12;CONFIG_FILE variable =BOOTLDR variable does not existConfiguration register is 0x2102
If no `boot system` is configured, the router boots the first valid image it finds in flash. That's usually fine but leaves you at the mercy of alphabetical order. Explicitly configure `boot system` on any device you care about — it prevents accidentally booting the wrong image.
Never reload a production router without a rollback plan. If the new image doesn't boot, you need physical or console access to recover via ROMmon. Remote-only upgrades on routers 500 km away are a career-limiting move.
The configuration register (`confreg`) controls boot behavior. `0x2102` is the normal value — load from flash and use NVRAM config. `0x2142` tells the router to ignore startup-config at boot — that's how password recovery works. Changing the confreg to 0x2142, reloading, and copying startup-config back to running-config is the standard IOS password recovery flow.
Section 5 wraps. You can navigate, configure, secure, and manage IOS devices. Section 6 puts it all together with VLANs and trunking.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.