Cisco IOS Files Explained

Cisco devices usually use integrated flash memory to store IOS software images and other files (e.g. backup configuration files). The startup configuration is stored in NVRAM. For each memory device available on the device, a file system called IOS file system (IFS) is created. To list file systems available on your device, the show file systems EXEC mode command is used:

R1#show file systems
File Systems:

       Size(b)       Free(b)      Type  Flags  Prefixes
*    255744000     221896413      disk     rw  flash0: flash:#
        262136        255005     nvram     rw  nvram:

In the command above you can see that I have two file systems available on my device. Here is a description of each field:

  • Size(b) – total memory in the file system (in bytes)
  • Free(b) – free memory in the file system (in bytes)
  • Type – type of the file system. disk represents the flash memory, and nvram represents the NVRAM memory
  • Flags – filesystem permissions. rw means that the file system is read/write.
  • Prefixes – file system aliases.


The IOS image is a single file that is loaded into RAM when the device boots. This file is usually stored in the flash memory, since this type of memory will retain the stored files even after the router is powered off or rebooted. To show the contents of the flash memory, the show flash: command is used:

R2#show flash:

System flash directory:
File  Length   Name/status
  3   33591768 c1900-universalk9-mz.SPA.151-4.M4.bin
  2   28282    sigdef-category.xml
  1   227537   sigdef-default.xml
[33847587 bytes used, 221896413 available, 255744000 total]
249856K bytes of processor board System flash (Read/Write)

The file c1900-universalk9-mz.SPA.151-4.M4.bin is an IOS image for the 1900 Integrated Services Router series. The other two XML files are used for encryption purposes.

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