Cisco Routers – Why Boot-Sequence is Stored in NVRAM by Default

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If the IOS of a cisco router is stored in flash, why then is the default behaviour (config reg: 0x2102) to get the boot-sequence from NVRAM instead of flash?

This source states that during runtime, flash on the router is ROM. So the only reason I can see to put the boot-sequence in NVRAM is to be able to edit it during runtime, but is that ever really necessary? You would have to reboot to make it effective anyway. So what is gained by making this the default?

Is manually changing the boot-sequence something that happens often in practice?

Best Answer

The flash is where the IOS image is stored in a compressed file (.bin). The NVRAM is the uncompressed file from Flash.

Changing the configuration register is sometimes needed in the case of the password recovery process. It can also be used for:

How the router boots (into ROMmon, NetBoot)
Options while booting (ignore configuration, disable boot messages)
Console speed (baud rate for a terminal emulation session)

Cisco's complete guide to config register commands/use on ALL routers: https://www.cisco.com/c/en/us/support/docs/routers/10000-series-routers/50421-config-register-use.html

Personally i've only made use of changing the config register in practice a few times, mostly password recovery on the old Cisco 2600 and 2800 Series Routers.

Once i had to fiddle about with the baud rate in order to send a file faster to a router via console.

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Here is a nice visual along with more information.

Best Answer

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