A Comparative Study of Architectural Impact on BGP Next-Hop Diversity
Large ISPs have been growing rapidly in both the size and global connectivity. To scale with the sheer number of routers, many providers have replaced the flat full-mesh iBGP connectivity with a hierarchical architecture, using either Route-Reflection (RR) or AS confederation. Given that each intermediate iBGP router in the hierarchy selects and propagates only one best path per destination network, there is a common perception that compared to full-mesh; hierarchical iBGP connectivity is likely to lose sight of alternative paths to external destinations. To gauge the path diversity reduction in the operational networks, the authors performed a comparative study by using iBGP data collected from two global-scale ISPs, with full-mesh core and RR architecture respectively.