Avoiding instability during graceful shutdown of OSPF

In this paper, we describe an enhancement to OSPF, called the IBB (I'll Be Back) capability, that enables other routers to use a router whose OSPF process is inactive for forwarding traffic for a certain period of time. The IBB capability can be used for avoiding route flaps that occur when the OSPF process is brought down in a router to facilitate protocol software upgrade, operating system upgrade, router ID change, AS and interface renumbering, etc. When the OSPF process in an IBB-capable router is inactive, it cannot adapt its forwarding table to reflect changes in network topology. This can lead to routing loops and/or black holes. We provide a detailed analysis of how and when loops or black holes are formed and propose solutions to prevent them. Using the GateD platform, we have developed an IBB extension to OSPF incorporating these solutions. Using this system in an experimental setup, we demonstrate that the overhead of the IBB extension is modest compared to the benefit it offers, and has good scaling behavior in terms of network size and the number of routers with inactive OSPF processes.