In addition to periodic updates, each agent also sends triggered updates; it does this whenever the forwarding tables in the node change. This occurs either due to changes in the topology, or because an agent at the node received a route update, and recomputed and installed new routes.
Each agent employs the split horizon with poisoned reverse mechanisms to advertise its routes to adjacent peers. ``Split horizon'' is the mechanism by which an agent will not advertise the route to a destination out of the interface that it is using to reach that destination. In a ``Split horizon with poisoned reverse'' mechanism, the agent will advertise that route out of that interface with a metric of infinity.
Each DV agent uses a default preference_ of 120. The value is determined by the class variable of the same name.
Each agent uses the class variable INFINITY (set at 32) to determine the validity of a route.
Tom Henderson 2011-11-05