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, a hierarchical iBGP connectivity is likely to lose sight of alternative paths to external destinations. To gauge the path diversity reduction in the operational networks, we performed a comparative study by using iBGP data collected from two global-scale ISPs, with full-mesh core and RR architecture respectively. Our results show that both ISPs suffer a significant reduction (up to 42%) in the overall path diversity. However the specifics of different iBGP architectures only made a minor impact (less than 2.9%) on this reduction. Rather, in both ISPs the majority of the alternative paths are eliminated by the first two criteria in BGP best path selection, i.e., LOCAL PREF and AS PATH length.