Doing don'ts: Modifying BGP attributes within an autonomous system

Internet Service Providers (ISPs) run the internal flavor of the Border Gateway Protocol (iBGP) for distributing routing information among border routers. While configuration languages allow routers to change iBGP attributes as a BGP message travels within the ISP's network, most prior work neglected this possibility, focusing only on the common case where iBGP attributes are left untouched. In this paper we aim at understanding what are the pros and cons of changing iBGP attributes. We estimate how many ISPs change iBGP attributes, and we motivate such a practice by showing usage scenarios where modified iBGP attributes yield better traffic engineering. We also revisit a well-studied problem in iBGP, that is, routing stability. We show that changing iBGP attributes can generate routing oscillations which are not possible otherwise, and are not detectable by state-of-the-art algorithms. We present a technique to check for routing oscillations even when iBGP attributes are changed, and we give simple guidelines for changing iBGP attributes while preserving stability.