IOS Upgrade

[Theofilos Tzachristas]

Hello all, 

I am trying to update the IOS version on a Cisco 2921 Router. It seems that I am not able to add a later IOS on flash: since it seems that there is enough disk space.  The show flash: result indicates that there isn't much available disk space.

#show flash:
-#- --length-- -----date/time------ path
450 14018 Aug 8 2014 21:55:56 +01:00 startup-config
452 109294244 Nov 21 2016 15:11:06 +00:00 c2900-universalk9-mz.SPA.156-3.M0a.bin
453 192328 Sep 4 2014 11:38:04 +01:00 crashinfo_20140904-113805-LON
456 47446 Aug 12 2015 22:58:38 +01:00 tfpt

47935488 bytes available (208551936 bytes used)

I can see that the IOS loaded on the machine is 15.2(4), so I assume it should be somewhere on the flash:

#show version
Cisco IOS Software, C2900 Software (C2900-UNIVERSALK9-M), Version 15.2(4)M3, RELEASE SOFTWARE (fc2)

So my questions are:

- How can I clear the flash: space that is not in use

- Why 15.2.4 is loaded and not the one displayed above 15.6. 

- Can I somehow validate the 15.6 in order to ensure it is not corrupted?

I know that I can erase the flash: but I am trying to understand how things are actually working. 

[balaji.bandi]

High level, Cisco Devices work with IOS software,  it is designed to operate if anything happens, Cisco software collects crash logs so the developer or software engineer can investigate the issue to fix.

the crash log can be anyway, malfunction, unexpected power loss, the behavior of demons or services.

You like to know about and investigate, best analysis and support can be provided by TAC people. and there are white papers around the cisco site also, not that I have in to provide now.

same like any other software - windows, mac os, android, the only internal team can only tell you behind the process what happend in deep level.