Every cisco routers has a configuration register which is saved in NVRAM and is a 16 bit value.
This post will not tackle all the 16 bits of the configuration register, but only the 13th bit which is used to either load IOS or ROMMON. Another post will be made to detail all the 16 bit configuration register.
Before continuing, it is important to understand the basic “boot process” of a router. When you power a router on, it first performs a POST, then loads the bootstrap program from ROM to RAM, which in return loads the appropriate IOS (bootstrap can load an IOS from tftp)/ROMMON or RXBOOT. Once loaded, the bootstrap program gives the hand to the IOS to handle the commands from there on.
The most important bit is the low order bit which is (2 for IOS, 1 for Rxboot and 0 for ROMMON)
Please note that by default, the low order boot bit is 2 thus 0x 2102
home-booth(config)#conf t home-booth(config)#config-register 0x2100 home-booth(config)#do wr Building configuration… [OK] home-booth(config)#do reload
Proceed with reload? [confirm]
(… a bit later)
%SYS-5-RELOAD: Reload requested by console. Reload Reason: Reload Command. System Bootstrap, Version 12.3(8r)T8, RELEASE SOFTWARE (fc1) Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.
rommon 1 > System Bootstrap, Version 12.3(8r)T8, RELEASE SOFTWARE (fc1) Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.
rommon 1 > rommon 1 > confreg 0x2102 rommon 2 >
**What could ROMMON be useful for? **Simple… restoring a router with a corrupted/broken IOS image!