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I have Cisco 857W and I need to upgrade IOS. But if I do that, I will not be able to log into it using Telnet; I would need a console cable which I don't have now.

How can I upgrade Cisco IOS without losing my configurations?

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Upgrading IOS does not result in configuration loss. Uploading a new version of IOS to a router does not even necessarily mean that it will even be the version loaded next reboot.

The first step is to copy the new firmware to the device.

copy tftp:10.1.1.1:/c857-k9o3y6-mz.124-12.T2.bin flash:

Assuming it has enough room to hold two IOS versions concurrenty, then you can modify the configuration to boot either one. In the config, you should have something like this:

boot-start-marker
boot system flash c857-k9o3y6-mz.123-11.T5.bin
boot-end-marker

If you were installing 124-12-t2 for example, you could have it try to boot this first, then fall back to the current version if it fails for any reason:

boot-start-marker
boot system flash c857-k9o3y6-mz.124-12.T2.bin
boot system flash c857-k9o3y6-mz.123-11.T5.bin
boot-end-marker

The startup-config is stored separately to the boot image, so does not get wiped.

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