EIGRP DUAL FSM (Finite State Machine)

This topic is to discuss the following lesson:

https://networklessons.com/cisco/ccie-routing-switching-written/eigrp-dual-fsm-finite-state-machine/

Hi Rene,

What do you mean by last reply and distance to destination increased ? I have ambiguity about this ? can you explain them in more detail to me please ?

Best Regard
Hussein Samir

Hello Hussein.

IE7 is an input event of any kind except for the following two kinds:

  1. a last reply input event.
  2. a distance to destination increased input event.

I hope this has been helpful.

Laz

Hi Rene,

“R1 expects a query from R2 so it sets its reply status flag to 1.”

Shouldn’t it be: “R1 expects a reply from R2 so it sets its reply status flag to 1.”?

Regards,
LP

Hi LP,

That’s what it should be yes. I just fixed it. Thanks for letting us know!

Rene

I am having a very hard time understanding this concept with the demonstrated diagram.
My main problem is not seeing a defined path (to my lack of understanding) as to the Input Events and their corresponding state.
I understand the overall goal of DUAL and how it functions (to some degree), but when presented the diagram I don’t know what it means to have those 4 different “types?” of ACTIVE states, and what is the structure of the Input Events.

Are the numbers a representation of the process of each one? or is it just one of the Input events that changes the state from Passive to Active (and vise versa).

How are the Query and Reply flags structured?

I guess I am over thinking this a lot, and the diagram might just be making it more complicated than it is, but I figured I could submit the question.

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!

Laz

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Thank you Laz. This cleared some of my questions. I took a look at the RFC and it turned out more confusing (the diagram) but the descriptions for each input event helped.

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Hi,

Just curious… to what extent would knowledge on this topic be expected for the 300-410 ENARSI exam?

I find it to be quite a deep topic, and would like to know how much weight to put into learning all of the possible states & Input Events. But I’m not sure if it may instead be the case that just a general understanding of the DUAL FSM process is needed & I can relax on the specifics.

Obviously with it being so early since the exam launched, there is very limited documentation out there so I was wondering if any of you has got any input on this?

Cheers,
Curtis.

Hello Curtis

It is highly unlikely that you will be asked about any detailed information about the FSM. I believe that it is best to understand the theory involved which includes much of the text above the FSM diagram in the lesson. Understand what active and passive states mean, and under what circumstances, in general, you switch to active, and when you reestablish the passive state. Understanding the four active states, memorizing the R and O flags, and knowing all of the input events I believe goes beyond what they’re looking for.

I hope this has been helpful!

Laz

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