Problem

This appears to be Cisco bug CSCui54490.

After a write erase (or, I believe, a format flash: or a PW recovery in this case?) a 3850 will ignore its startup-config and always boot a blank configuration.

My switch is running IOS-XE 16.12.11 (Gibraltar), date Sun 31-Mar-24 12:30 (a minor version behind at the time of writing), though this seems to be a long-standing issue.

show version output suggests config register is 0x142.

Switch#sh ver | inc register
Configuration register is 0x142 (will be 0x102 at next reload)

Solution

In config mode, set, then unset, the ignore startupconfig boot variable (SWITCH_IGNORE_STARTUP_CFG).

Switch(config)#system ignore startupconfig switch all
Applying config on Switch 1...[DONE]
Switch(config)#no system ignore startupconfig switch all
Applying config on Switch 1...[DONE]
Switch(config)#end
Switch#reload

Once the switch comes back up, verify that it loaded its config and the configuration register is now 0x102:

3850-48t-lab-0#sh ver | inc register
Configuration register is 0x102

If you loaded the startup config and then wr’d during troubleshooting, you may need to regenerate your SSH host keys with crypto key generate rsa mod x.

You can determine if this is necessary by quickly reviewing the show ip ssh output:

3850-48t-lab-0#sh ip ssh
SSH Enabled - version 1.99

Once this boot variable is set, the switch should load its startup-config properly (until you wr erase it again).

Attempted fixes

• Set config-register 0x102 and reload (no change)
• Set config-register 0x2102 and reload (no change)
• Unset SWITCH_IGNORE_STARTUP_CFG in rommon (I did not set SWITCH_IGNORE_STARTUP_CFG first, this may be why this did not work)