10 - Measuring the shared fate of IGP engineering: considerations and take-away

Summary

The Internet is made of many separate routing domains called Autonomous Systems (ASs), each of which runs an IGP such as IS–IS or OSPF. The IGP handles routes to destinations within the AS, but does not calculate routes beyond the AS boundary. Internet Gateway Protocol engineering (or traffic engineering or IGP optimization) is the tuning of local IS–IS or OSPF metrics to improve performance within the AS. Today, IGP engineering is an ad-hoc process where metric tuning is performed by each AS in isolation. That is, each AS optimizes paths within its local network for traffic traversing it without coordinating these changes with neighboring ASs. The primary assumption behind such an assertion is that there is sufficient separation between intra-domain and inter-domain routing.

Beyond the AS boundary, the choice of AS hops is determined by the BGP,; BGP engineering is a less developed and less understood process compared to IGP engineering. In addition to whether there is a physical link between two ASs over which routes and traffic can flow, there are several BGP policies that determine which inter-domain paths are exposed to a neighboring AS. Business peering policies can directly translate into which routes are exported to each AS. After all these policies are applied, the remaining feasible paths are subjected to the “hot-potato” routing policy. Hot-potato routing occurs when there are multiple egress points to reach a destination.