BGP Add-Paths: The Scaling/Performance Tradeoffs

Internet Service Providers design their network with resiliency in mind, having multiple paths towards external IP subnets available at the borders of their network. However, with the current internal Border Gateway Protocol, BGP routers and route reflectors only propagate their (unique) best path over their iBGP sessions. As a result, at the BGP router level, path diversity tends to be poor. Such lack of path diversity can lead to MED oscillations, prevents an efficient use of multipath BGP and does not allow for a fast and local recovery upon nexthop failure. Advertising multiple paths over iBGP sessions with BGP Add-Paths solves those issues, depending on the way the additional paths are selected. In this paper, we analyze the various options for the selection mode of the paths to be advertised. We show that these modes differently fulfill the needs of Add-Paths applications such as fast recovery upon failure and MED oscillation avoidance. We also show in our analysis that the costs and benefits bound with these modes depend on the connectivity of the AS where it is deployed. To support the analysis, we developed a tool allowing to measure the scaling of these modes in a given network. We illustrate the utilization of this tool on synthetic Internet topologies, and provide some recommendations for the choice of an Add-Paths selection mode.