Operation of Cisco IOS software

Operation of Cisco IOS software
2.1.5 This page will introduce the three distinct operating environments, or modes, of Cisco IOS devices:


The Cisco IOS devices have three distinct operating environments or modes:

• ROM monitor

• Boot ROM

• Cisco IOS

At startup, a Cisco router normally loads into RAM and executes one of these operating environments. A system administrator can use the configuration register setting to control the default startup mode for a router.

The ROM monitor performs the bootstrap process and provides low-level functionality and diagnostics. It is used to recover from system failures and to recover a lost password. The ROM monitor cannot be accessed through any of the network interfaces. It can only be accessed by way of a direct, physical connection through the console port.

When the router is running in boot ROM mode, only a limited subset of the Cisco IOS feature set is available. Boot ROM allows write operations to flash memory and is used primarily to replace the Cisco IOS image that is stored in flash. The Cisco IOS image can be modified in boot ROM with the copy tftp flash command. This command copies an IOS image that is stored on a TFTP server into the flash memory of a router.

The normal operation of a router requires use of the full Cisco IOS image as stored in flash. In some devices, the IOS is executed directly from flash. However, most Cisco routers require a copy of the IOS to be loaded into RAM and also executed from RAM. Some IOS images are stored in flash in a compressed format and have to be expanded when copied to RAM.

To see the IOS image and version that is running, use the show version command, which also indicates the configuration register setting. The show flash command is used to verify that the system has sufficient memory to load a new Cisco IOS image.

This page concludes the discussion about Cisco IOS. The next lesson will describe the initial startup of Cisco routers.