Cisco IOS Boot Process
This exercise assumes using Packet Tracer 6.x
When you boot a CISCO switch or Router, it goes through a standard load sequence, using something that looks like a BIOS (ROMMON) to load something that looks like a monolithic UNIX based OS. There are a range of reasons including maintenance, security and QA for why we need to be able to operate at this level.
Create a new configuration file in Packet Tracer called “SecuringIOS” or use real hardware!
Create a 2911 router and power it up.
Open the CLI.
The router will run power on self-tests (POST) from ROM.
On power up, bootstrap code loads from ROM and a mini-IOS called RXBOOT is loaded with the purpose of loading the full version of IOS.
Document the bootstrap version, identify the router model, processor type and memory. The version of IOS in Packet Tracer is quite old; more information is available in current equipment.
If POST succeeds, the router will look for a valid IOS to load from flash memory. The image will be decompressed (and will display #### symbols whilst doing so) and loaded.
Document the IOS version and the hardware configuration of the router.
After boot, a pre-configuration file called startup-config will be copied from NVRAM to RAM and called running-config.
In a large-scale environment where many switches are being configured, we can use DHCP to orchestrate new switches and set configurations. We are not going to cover that here, do some research and reading now in the reference manuals for the router.
If no valid startup-config file is found, the router will broadcast for a TFTP server looking for a start-up file. This can be used by us as administrators to auto-configure devices, but it is also a potential attack vector for the bad guys! If you are not familiar with TFTP, do some research and reading now in the reference manuals for the router.
If there is still no configuration available, the router will go into a set-up mode.
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