Router# write erase\nErasing the nvram filesystem will remove all configuration files! Continue? [confirm]\n[OK]\nErase of nvram: complete\n<\/code><\/pre>\nNote: Both write erase<\/code> and erase startup-config<\/code> execute the identical internal procedure on modern Cisco IOS and IOS-XE platforms.<\/em><\/p>\n<\/span>Step 4: Clear Switchport and Cryptographic Artifacts<\/span><\/h3>\nOn modular routers equipped with integrated EtherSwitch modules (such as Cisco 800, 1900, or ISR 4000 series with NIM-ES2 modules), VLAN database definitions reside in Flash rather than NVRAM:<\/p>\n
Router# delete flash:vlan.dat\nDelete filename [vlan.dat]? \nDelete flash:\/vlan.dat? [confirm]\n<\/code><\/pre>\n<\/span>Step 5: Reload Without Saving<\/span><\/h3>\nReboot the router using the reload<\/code> command. When Cisco IOS prompts to save the modified running configuration, enter no<\/code><\/strong>. Entering yes<\/code> writes the active configuration from DRAM back into NVRAM, immediately overriding the erase command.<\/p>\nRouter# reload\nSystem configuration has been modified. Save? [yes\/no]: no\nProceed with reload? [confirm]\n<\/code><\/pre>\n<\/span>Step 6: Verify Factory Default Prompt<\/span><\/h3>\nUpon rebooting, confirm that the router prompts for initial setup:<\/p>\n
--- System Configuration Dialog ---\n\nWould you like to enter the initial configuration dialog? [yes\/no]: \n<\/code><\/pre>\nType no<\/code> to access the default unconfigured command line (Router><\/code>).<\/p>\n
\n<\/span>Resetting Without a Password via ROMMON Mode<\/span><\/h2>\nIf administrative credentials are forgotten, corrupted, or unavailable, you must bypass the startup configuration by modifying the software configuration register in ROMMON mode via a physical console connection.<\/p>\n
<\/span>Step 1: Connect to the Physical Console Port<\/span><\/h3>\nConnect an RJ-45-to-DB9 rollover cable or a standard USB console cable between your management workstation and the router’s physical CONSOLE<\/code> port. Configure your terminal emulator (e.g., PuTTY, Tera Term, SecureCRT) to the standard Cisco serial settings:
\n* Baud Rate:<\/strong> 9600 bps
\n* Data Bits:<\/strong> 8
\n* Parity:<\/strong> None
\n* Stop Bits:<\/strong> 1
\n* Flow Control:<\/strong> None<\/p>\n<\/span>Step 2: Send the Break Sequence<\/span><\/h3>\nPower-cycle the router (turn the physical power switch off and on, or unplug the power cable). Within the first 60 seconds of hardware initialization, send the serial Break<\/code> sequence from your terminal emulator:
\n* PuTTY:<\/strong> Right-click the title bar $\\rightarrow$ Special Command<\/strong> $\\rightarrow$ Break<\/strong> (or press Ctrl<\/code> + Break<\/code>).
\n* Tera Term:<\/strong> Press Alt<\/code> + B<\/code>.
\n* macOS \/ Linux (screen<\/code> \/ minicom<\/code>):<\/strong> Press Ctrl<\/code> + A<\/code>, then F<\/code> (or Ctrl<\/code> + \\<\/code>).<\/p>\nThe router halts normal boot execution and displays the ROMMON command prompt:<\/p>\n
System Bootstrap, Version 15.4(3r)S, RELEASE SOFTWARE\nCopyright (c) 2015 by cisco Systems, Inc.\n\nrommon 1 >\n<\/code><\/pre>\n<\/span>Step 3: Change the Configuration Register to Bypass NVRAM<\/span><\/h3>\nInstruct the bootstrap loader to ignore the NVRAM configuration during boot by setting the register to 0x2142<\/code>:<\/p>\nrommon 1 > confreg 0x2142\nYou must reset or power cycle for new config to take effect.\nrommon 2 > reset\n<\/code><\/pre>\n(On older legacy models, the command is o\/r 0x2142<\/code> followed by i<\/code> or boot<\/code>.)<\/em><\/p>\n<\/span>Step 4: Boot to Default Prompt and Erase Configuration<\/span><\/h3>\nThe router boots without loading the existing startup-config<\/code>. When prompted:<\/p>\nWould you like to enter the initial configuration dialog? [yes\/no]: no\nRouter> enable\nRouter# write erase\nErasing the nvram filesystem will remove all configuration files! Continue? [confirm]\n[OK]\nErase of nvram: complete\n<\/code><\/pre>\n<\/span>Step 5: Reset the Configuration Register to Normal<\/span><\/h3>\nTo ensure the router loads its new configuration on subsequent reboots, you must change the register back to 0x2102<\/code>:<\/p>\nRouter# configure terminal\nRouter(config)# config-register 0x2102\nRouter(config)# end\nRouter# write memory\n[OK]\n<\/code><\/pre>\n<\/span>Step 6: Reload into Factory State<\/span><\/h3>\nExecute reload<\/code> to confirm the router initializes cleanly under the default register:<\/p>\nRouter# reload\nProceed with reload? [confirm]\n<\/code><\/pre>\n
\n<\/span>Configuration Register Settings (0x2102 vs. 0x2142)<\/span><\/h2>\nThe software configuration register is a 16-bit value stored in NVRAM that directly controls how the processor initializes, where it loads the Cisco IOS software from, and whether it parses the startup configuration.<\/p>\n
<\/span>What is the functional difference between 0x2102 and 0x2142?<\/span><\/h3>\n\n0x2102<\/code> (Standard Operational Value):<\/strong> The default enterprise setting. Bit 6 is set to 0<\/code>, instructing the bootloader to read and execute the startup-config<\/code> stored in NVRAM. Bits 0\u20133 (boot field) are set to 0x2<\/code>, telling the router to load the first valid Cisco IOS image located in Flash.<\/li>\n0x2142<\/code> (Password Recovery \/ Bypass Value):<\/strong> Bit 6 is toggled to 1<\/code> (representing 0x0040<\/code> in hex). This tells the bootstrap program to completely ignore NVRAM during startup, simulating a blank configuration regardless of what is stored in memory.<\/li>\n<\/ul>\n<\/span>Why does my Cisco router keep its old configuration after running write erase<\/code>?<\/span><\/h3>\nThis condition occurs for one of two common reasons:
\n1. Accidental Configuration Overwrite:<\/strong> When issuing the reload<\/code> command, Cisco IOS prompts: System configuration has been modified. Save? [yes\/no]:<\/code>. If the administrator enters yes<\/code>, the current running-config<\/code> in DRAM is written right back to NVRAM, replacing the wiped file.
\n2. AutoInstall \/ DHCP Boot Options:<\/strong> If connected to a live network with active DHCP servers delivering TFTP boot paths (DHCP Option 67), an unconfigured Cisco router automatically attempts to download a network configuration file across the wire via Cisco AutoInstall.<\/p>\n<\/span>Why does the router lose configuration changes after every reboot?<\/span><\/h3>\nIf you can configure interfaces and save settings using write memory<\/code>, but find the router completely unconfigured after every power cycle, the configuration register is still set to 0x2142<\/code>. Because NVRAM is bypassed on every boot, changes written to NVRAM are never loaded into DRAM.<\/p>\nTo verify your active and upcoming register settings, execute:<\/p>\n
Router# show version | include register\nConfiguration register is 0x2142 (will be 0x2102 at next reload)\n<\/code><\/pre>\nIf the register is not scheduled to change, update it:<\/p>\n
Router# configure terminal\nRouter(config)# config-register 0x2102\nRouter(config)# end\nRouter# write memory\n<\/code><\/pre>\n