After you have done your initial research and have narrowed down your choices but before you have made a final decision on which provider or providers to use, you will probably want to see each provider’s service-level agreement (SLA). The SLA, as we mentioned in Chapter 7, Traffic Shaping, is the official commitment between the ISP and the client. This document defines particular aspects of the service that should be provided to the client. An ISP is not required to have an SLA. Nevertheless, many providers have an SLA, and many that do make their SLA available on their website.

Although the SLA might be part of the contract between an ISP and their clients, it might not be part of it. There are several aspects of service that can be defined by the SLA. These can include throughput, mean time between failures, mean times between recovery, mean times between repair, percentage of up-time, and much more. It can also specify which party is responsible for recording faults and paying fees. It may also delineate the procedure for supervising/monitoring performance levels, who to contact when there are issues, and finally, the time frame in which problems will be resolved. Typically, an SLA will also specify the consequences for an ISP not fulfilling its SLA obligations, and in some cases, if the service provider does not abide by its obligations, the client may have the right to terminate the contract, or at least receive a refund for the time during which connectivity is not available.

Redundancy and high availability for internet connections is often more important in a business environment, and this is the most typical scenario in which you will be configuring a multi-WAN setup. In such cases, you may consider acquiring a T1 service, since such connections have been held in high regard for a number of years. In general, the SLA for a T1 connection is more favorable than SLAs for other types of connections. Moreover, a T1 service is seen as more reliable. With pfSense, however, you needn't necessarily lease a T1; you can achieve high levels of reliability at a lesser cost via a multi-WAN setup. Two low-cost broadband connections can provide greater bandwidth and the same or greater level of reliability than a single T1 connection.