EIGRP DUAL FSM (Finite State Machine)

Hello Hugo

The truth is that the EIGRP finite state machine is indeed complicated, so don’t feel bad that you cannot fully get your head around it. Keep in mind that a detailed knowledge of this process is not necessary for the certifications, and is included here only for completeness. The important thing to realize here is that a route’s state in a converged network is passive as long as the feasibility condition is satisfied. Various events can take place that will move a route from passive into an active state, and that a route can move from one active state to another based on the events that occur. There is no need to be able to follow the full process from start to finish, and rarely (if ever) will you use debugs to the level of detail shown in the diagram for troubleshooting.

The four different types of active states simply means that we can have four different combinations of R and O flags that correspond to an active state. For all four active states, R=1. This flag simply states that a query has been sent, and a reply is expected. A route will not go into passive state again until a reply has been received for all queries sent. R will remain 1 until a reply has been received from all queries sent to neighbouring routers.

The O flag has four values, either 0, 1, 2, or 3. Each one corresponds to a different input event, and this is what differentiates between the active states. Specifically the O flags are defined as:

0 = metric increase during ACTIVE state
1 = node i originated
2 = QUERY from, or link increase to, successor during ACTIVE state
3 = QUERY originated from successor

The details of what those actually are, are outside the scope of the lesson, and the certification exam. However, if you’re interested, you can find out more directly from the EIGRP RFC 7868.

I hope this has been helpful!