My prior post about SLAs described calculation methodologies. But what data must be collected for those calculations and what's a useful calculation methodology?
A business with high availability requirements may prefer an SLA that measures connectivity availability. This business must determine the required connectivity and determine how to measure it. One method of measurement might be counting lost pings between sites or monitoring with IPSLA (Cisco specific). This type of business typically has a highly redundant network, so measuring SLA based on devices and links isn't very useful. The monitoring system needs to run tests over the important communication paths and record all connectivity failures. Totaling the failures across the network would yield the availability SLA. Financial organizations typically build these types of networks.
A different organization that has less stringent connectivity requirements may use the same monitoring methodology, but averages the availability measurements across all sites. This business will typically have the ability for remote sites to operate while disconnected, which is why averaging network downtime across sites is an acceptable measurement. There can be significant savings in equipment and links in this business model. A manufacturing business may use this model, where remote sites can run independently for a few hours or a few days.
Device-level SLAs might be applicable in parts of an organization and the networking team needs to decide if it is best to average device downtime or to sum the total downtime, or both. A food packaging company might use a device-centric SLA, summing all downtimes for devices that are key to the processing, packaging, and shipping process. Many of these companies cannot shut down their processing lines without significant cost. For example, jelly-bean like products can solidify in the piping if the production line is shutdown. Even a failure to move products off the loading dock can cause a backup that causes a production line to shutdown, so the failure of a network connection to the shipping label printer can be catastrophic.
The last example above demonstrates how a single SLA is seldom possible or desirable. Businesses may need to have multiple SLAs, one for each major facet of the business.
-Terry