When selecting or upgrading a Cisco IOS router, it is important to choose the proper IOS image with the correct feature set and version. The Cisco IOS image file is based on a special naming convention. The name for the Cisco IOS image file contains multiple parts, each with a specific meaning. It is important to understand this naming convention when upgrading and selecting a Cisco IOS Software.

As shown in Figure 1, the show flash command displays the files stored in flash memory, including the system image files.

An example of an IOS 12.4 software image name is shown in Figure 2.

Figure 3 illustrates the different parts of an IOS 15 system image file on an ISR G2 device:

The most common designation for memory location and compression format is mz. The first letter indicates the location where the image is executed on the router. The locations can include:

The compression format can be either z for zip or x for mzip. Zipping is a method Cisco uses to compress some run-from-RAM images that is effective in reducing the size of the image. It is self-unzipping, so when the image is loaded into RAM for execution, the first action is to unzip.

Note: The Cisco IOS Software naming conventions, field meaning, image content, and other details are subject to change.

Memory Requirements

On most Cisco routers including the integrated services routers, the IOS is stored in compact flash as a compressed image and loaded into DRAM during boot-up. The Cisco IOS Software Release 15.0 images available for the Cisco 1900 and 2900 ISR require 256MB of flash and 512MB of RAM. The 3900 ISR requires 256MB of flash and 1GB of RAM. This does not include additional management tools such as Cisco Configuration Professional (Cisco CP). For complete details, refer to the product data sheet for the specific router.