I have a 1821 SDM. I recently changed ISP's and tried to reconfigure the router with my new set of IP's. No when ever I reboot the router, it resets back to the old config. I have tried everything I know. Anyone have any ideas?
Do make sure that your config register value is set to 0X2102.
I feel your config register value is set to 0X2142 which will result in what you are experincing at present.
You can verify the same using show version command...
Once you are sure that its set as 0X2142 do change it using config-register 0X2102 and save the config.
After saving the config do check with show version command for proper updation try with a reset and check for the new register value..
Did you saved your running-config to startup-config