Routing Protocols: EIGRP Concepts

EIGRP Concepts

Successor

A successor for a particular destination is a next hop router that satisfies these two conditions:
it provides the least distance to that destination
it is guaranteed not to be a part of some routing loop

The first condition can be satisfied by comparing metrics from all neighboring routers that advertise that particular destination, increasing the metrics by the cost of the link to that respective neighbor, and selecting the neighbor that yields the least total distance. The second condition can be satisfied by testing a so-called Feasibility Condition for every neighbor advertising that destination. There can be multiple successors for a destination, depending on the actual topology.

The successors for a destination are recorded in the  topology table and afterwards they are used to populate the routing table as next-hops for that destination.


Feasible Successor

A feasible successor for a particular destination is a next hop router that satisfies this condition:
it is guaranteed not to be a part of some routing loop

Thus, every successor is also a feasible successor. However, in most references about EIGRP the term "feasible successor" is used to denote only those routers which provide a loop-free path but which are not successors (i.e. they do not provide the least distance). From this point of view, for a reachable destination there is always at least one successor, however, there might not be any feasible successors.
Active and Passive State

A destination in the topology table can be marked either as Passive or Active. A Passive state is a state when the router has identified the successor(s) for the destination. The destination changes to Active state when current successor no longer satisfies the Feasibility Condition and there are no feasible successors identified for that destination (i.e. no backup routes are available).


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

A Feasible Distance (FD) is the lowest known distance from a router to a particular destination.

This diagram shows the example of Feasible and Reported distance of EIGRP network topolgy description as above

*Feasibility Condition

If, for a destination, a neighbor router tells us that it is closer to the destination than we have ever been, then this neighbor lies on a loop-free route to this destination.
In exact terms, every neighbor that satisfies the relation RD < FD for a particular destination is on a loop-free route to that destination.