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:
Ciscozine(config)#warm-reboot Warm reboot will be possible after the next power cycle or reload. Ciscozine(config)#
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) Statistics: 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 Ciscozine#
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 Ciscozine#
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 Ciscozine#
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.