Speed up your reload

How long does it take to reload your router? 3 or 4 minutes? Do you know that is possible to speed up your reboot? If your answer is negative, read how warm reload is faster than cold (classic) reload.

Introduced in Cisco IOS Release 12.3(2)T, the warm reload feature allows users to reload their routers without reading images from storage. That is, the Cisco IOS image reboots without ROM monitor mode (ROMMON) intervention by restoring the read-write data from a previously saved copy in the RAM and by starting execution without either copying the image from flash to RAM or self-decompression of the image. Thus, the overall availability of your system improves because the time to reboot your router is significantly reduced.

Additional memory is consumed because a copy of the initialized variables must be stored for a warm reboot to function. However, to consume as little memory as possible, a copy of the initialized variables is kept in a compressed form, which is marked as “read-only” to prevent corruption.

To enable the warm reload, type the commnad ‘warm-reboot’, save the conf and reload the system to enable the functionality:

Warm reboot will be possible after the next power cycle or reload.

After that, you can see how many bytes are used by warn-reload feature:

Ciscozine#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 5 (min)


0 warm reboots due to crashes and 0 warm reboots due to requests have taken
place since the last cold reboot
5922 KB taken up by warm reboot storage


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

Ciscozine#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
 0x40019990  0x430DFFFF    51144304  IText  R/O    main:text
 0x430EA0C0  0x44EF631F    31507040  IData  R/W    main:data
 0x44EF6320  0x455E01FF     7249632  IBss   R/W    main:bss
 0x455E0200  0x45BA80D3     6061780  Local  R/W    main:saved-data
 0x45BA80D4  0x4F3FFFFF   159743788  Local  R/W    main:heap
 0x80000000  0x8F3FFFFF   255852544  Local  R/W    main:(main_k0)
 0xA0000000  0xAF3FFFFF   255852544  Local  R/W    main:(main_k1)

Free Region Manager:

      Start         End     Size(b)  Class  Media  Name


To reload the router with warm restart use the command ‘reload warm’!

In my test, the reload time of a Cisco 2811 router booting from flash was reduced from 125 to 47 seconds as measured by the %SYS-6-BOOTTIME: Time taken to reboot after reload … syslog message.

With cold reload

*Apr 29 11:13:16.147: %SYS-6-BOOTTIME: Time taken to reboot after reload =  125 seconds

With warm reload

*Apr 29 11:16:30.199: %SYS-6-BOOTTIME: Time taken to reboot after reload =   47 seconds

The warm reload can also be used to upgrade/downgrade Cisco IOS. The warm upgrade feature provides the capability for a Cisco IOS image to read and decompress another Cisco IOS image and then transfer control to this new image. This functionality reduces the downtime of a device during planned Cisco IOS software upgrades or downgrades. With the introduction of the Warm Upgrade feature, packet forwarding is able to continue while the new Cisco IOS image is read and decompressed.

To perform a warm upgrade, use the ‘reload warm file’ command:

Ciscozine#reload warm file ?
flash:  device:file name to load
ftp:    device:file name to load
http:   device:file name to load
https:  device:file name to load
pram:   device:file name to load
rcp:    device:file name to load
scp:    device:file name to load
tftp:   device:file name to load


Remember: The device is down only when the current image is overwritten with the new image, and the new image loads and reconfigures the operating system.



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