ITIL is service centric. Everything that ITIL defines and proposes is based on the premise that services are well known, and are identified. But, imagine being asked to take over IT support without having the slightest clue on what services you are tending to.
This situation is not made-up. It happens. You may not have issues to begin with but when the shit hits the fan, your support starts to disintegrate.
OK. I don’t want to sound morose, but it is indeed true that if you do not know the services that you are working for, how could you do an accurate business impact when you change something, when your server stops working or when a switch fails in the dead of night.
ITIL defines a service catalog, which is basically like a menu that you get in a restaurant. The potential customer looks at it and says, I want this, this and this. In a situation, where you are taking over support from an incumbent, you probably will have to build one yourself (and charge the customer for it too).
If you don’t have a service catalog, but have a configuration management database (CMDB), the job is half done. You need to map the configuration items to the service catalog. In the ITIL Service Design publication, the technical service catalog is defined, which gives you an overview of services mapped with the CMDB.
When this has been mapped out, you could trace the configuration item (CI) back to any one or multiple services that you are offering to your customer. And, this setup places you in a perfect position to identify the business impact whenever you make IT changes. And, helps you troubleshoot incidents when a component fails.