Enhanced Interior Gateway Routing Protocol - Reported Distance and Feasible Distance

Reported Distance and Feasible Distance

Reported Distance (RD) is the total metric along a path to a destination network as advertised by an upstream neighbor. This distance is sometimes incorrectly referred to as Advertised Distance (AD) and is equal to the current lowest total distance through a successor for a neighboring router.

A Feasible Distance (FD) is the lowest known distance from a router to a particular destination. This is the Reported Distance (RD) + the cost to reach the neighboring router from which the RD was sent. It is important to note that this metric represents the last time the route went from Active to Passive state. It can be expressed in other words as a historically lowest known distance to a particular destination. While a route remains in Passive state, the FD is updated only if the actual distance to the destination decreases, otherwise it stays at its present value. On the other hand, if a router needs to enter Active state for that destination, the FD will be updated with a new value after the router transitions back from Active to Passive state. This is the only case when the FD can be increased. The transition from Active to Passive state in effect marks the start of a new history for that route.

For example, if the route to a newly discovered destination X went from Active to Passive state with a total distance of 10, the router sets the RD and FD to 10. Later this distance decreases from 10 to 8. The distance remains in the Passive state (because distance decrease never violates the Feasibility Condition) and the router updates the RD and FD to 8. Even later, the distance increases to 12 but in such a way that there is still a valid successor or feasible successor available. In this case, the RD gets updated to 12, however, the FD will remain at the value of 8. Therefore, the values of RD and FD can be different. Finally, the actual successor fails and no other feasible successor is currently identified. Therefore, the router has to transition to Active state and ask its neighbors for a new route to the destination X. Assuming that the newly found path to that destination has a total distance of 10, the router will transition back to Passive state and update both its RD and FD to the new shortest path length, in this case, 10.