The Warm Reload functionality introduced in IOS release 12.3(2)T significantly reduces the reload time. In my test lab, the reload time of a Cisco 2800 router booting from flash was reduced from 135 to 54 seconds as measured by the %SYS-6-BOOTTIME: Time taken to reboot after reload ... syslog message.

The theory behind warm reload is simple: the router saves initial data (as stored in IOS image) in a separate memory region and reuses saved data together with IOS code already residing in RAM to restart IOS. Of course, the IOS code (depending on platform's memory management capabilities) or saved data could get corrupted, therefore the warm reload cannot be used continuously (and the router falls back to traditional reload if the router crashes before a specified time interval).

Warm reload is configured with the warm-reboot count number uptime minutes configuration commands. After it has been configured, a router reload (or power-up) is needed to initialize the saved data region. When the warm reboot is operational (as verified with the show warm-reboot command), you can use reload warm command to start it. The output of the show warm-reboot command displays all the relevant setup parameters as well as the amount of memory used by this feature:

a2#show warm-reboot
Warm Reboot is enabled
Maximum warm reboot count is 5
Uptime after which warm reboot is safe in case of a crash is 2 (min)

Statistics:

0 warm reboots due to crashes and 0 warm reboots due to requests 
have taken place since the last cold reboot
2823 KB taken up by warm reboot storage

The saved data region is also displayed with the show region command:

a2#show region
Region Manager:

      Start         End     Size(b)  Class  Media  Name
 0x0F400000  0x0FFFFFFF    12582912  Iomem  R/W    iomem:(uncached_iomem_region)
 0x3F400000  0x3FFFFFFF    12582912  Iomem  R/W    iomem
 0x40000000  0x4F3FFFFF   255852544  Local  R/W    main
 0x4000F000  0x431DFFFF    52236288  IText  R/O    main:text
 0x431E0000  0x45F8C25F    47891040  IData  R/W    main:data
 0x45F8C260  0x465FFA5F     6764544  IBss   R/W    main:bss
 0x465FFA60  0x468C19AF     2891600  Local  R/W    main:saved-data
 0x468C19B0  0x4F3FFFFF   146007632  Local  R/W    main:heap
 0x80000000  0x8F3FFFFF   255852544  Local  R/W    main:(main_k0)
 0xA0000000  0xAF3FFFFF   255852544  Local  R/W    main:(main_k1)