Best-effort networks leverage Interior Gateway Protocol (IGP) technologies such as Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS) to determine paths for routing packets between hosts. IGP protocols use a Shortest Path First (SPF) algorithm to build routing tables. The routing engine references these routing tables at each router hop traversed by the packets.

Architects for these networks provide QoS by over-provisioning the links and routers so that network congestion does not introduce unwanted levels of latency, jitter, and packet loss. Overprovisioning a network would appear to be a simple way to provide the necessary level of QoS. However, while this approach requires a significant amount of added bandwidth, it still cannot guarantee service levels in an operational environment. Historically, to provide toll-grade services during predictable periods of network usage, service providers must over-provision bandwidth resources by a factor of between 10:1 and 20:1.

Best-effort networks are not ideal in dealing with temporary or permanent outages. IGP will advertise the outage and initiate a route table reconstruction based on the modified topology. This process is called route re-convergence, and if not designed carefully or optimized, can take seconds to stabilize. Web browsing or e-mail users do not typically notice these seconds. However, route re-convergence can be detrimental to VoIP users in the middle of a conversation, as it impacts both latency and jitter.

Source: Juniper Networks, Inc. White Paper