In order of preference/priority, what factors do you consider in driving an upgrade (or downgrade) with Cisco IOS? If no compelling factors exist, how long would you allow a particular version of IOS to stay running? I’ve seen some switches with uptimes > 5 years. And when upgrading, how is the specific IOS release identified as the upgrade target?
In order of preference/priority, best practice tends to dictate an upgrade based on these factors:
A device that is very critical to the infrastructure may not be as aggressively upgraded as one that is less critical. Consideration is given to the role of the device, the redundancy surrounding it, and the impact of the upgrade itself by the downtime incurred or by the possibility of having config feature behavior changes or different defaults when going between major versions. This is the necessity question that also touches on soft costs such as the time and resources to accomplish the upgrades measured against the weight given to each of the factors such as vulnerabilities.
A downgrade might be in order if:
In choosing the right target version for an upgrade,
There are certainly arguments for going to an ED or LD version, but the desire, of course, is to get to the most stable version that meets requirements.
Question originally posed and self-answered on networkengineering.stackexchange.com.