Researchers and network operators often say that BGP table transfers are slow. Despite this common knowledge, the reasons for slow BGP transfers are not well understood. This paper explains BGP table transfer delays by combining BGP messages collected at a large VPN provider backbone and controlled experiments with routers of three different vendors as well as a software BGP speaker. Our results show that table transfers both in the provider network and in the controlled experiments contain gaps, i.e., periods in which both the sending and receiving routers are idle, but no BGP routes are exchanged. Gaps can represent more than 90 % of the table transfer time. Our analysis of a software router and discussions with router vendors indicate that gaps happen because of the timer-driven implementation of sending of BGP updates. Hence, gaps represent an undocumented design choice that gives preference to more controlled router load over faster table transfers
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