Route prioritization: IS-IS

A router has a predetermined order for route installation. This is no longer sufficient, as you sometimes need to prioritize certain routes or prefixes over others for better convergence or to provide differentiated services. In a network with a large number of routes, it is sometimes important to control the order in which routes get updated due to a change in the network topology. For example, it would be useful to first update IGP routes that correspond to a BGP next hop, and only then update the rest of the IGP routes. At a system level, the router implements reasonable defaults based on heuristics to determine the order in which routes get updated. However, the default behavior is not always optimal. This feature introduces support for allowing the user to control the order in which the routes get updated from IS-IS to rpd and from rpd to kernel.