Selecting a Cisco IOS Image
When you select the Cisco IOS image for upgrade, consider the following:
Memory requirement—The router should have sufficient disk or flash memory to store the Cisco IOS. The router should also have sufficient memory (DRAM) to run the Cisco IOS. The recommended logging buffer in DRAM ranges from 8 kilobytes to 64 kilobytes. If the router does not have sufficient memory (DRAM), the router will have boot problems when it boots through the new Cisco IOS.
Interfaces and modules support—You must ensure that the new Cisco IOS supports all the interfaces and modules in the router.
Software feature support—You must ensure that the new Cisco IOS supports the features used with the old Cisco IOS.
Security image—ASR 901 does not support loading security images in the non-secure environment or node. Loading the security images affect the functionality.