GRE encapsulated tunnels have very low overhead and are best used for Best-Effort class of service. Packets within the GRE tunnel follow the Interior Gateway Protocol (IGP) shortest path from edge to edge. If a failure occurs within the service core network, the tunnel only converges as quickly as the IGP itself. If Equal Cost Multi-Path (ECMP) routing is used in the core, many loss-of-service failures can be minimized to sub-second timeframes.