6 Replies Latest reply: Jan 23, 2010 1:20 PM by Sal RSS

    Understanding EIGRP and packet tracer 5



      This is a two wuestions in one to save posting space. First question is about EIGRP. The professor of my CCNA class spent about 4-5 hours covering EIGRP and its mnechanics. However, i did not understand what was going on. I asked him some questions and im still fuzzy. I do not understand what the difference between Fesible distance and Reported distnace is. Also this whole Fesible condition thing confuses me. Second. I do not understand how Successor and Fesible successor are determined?



      Now in packet tracer i noticed thei lil icon in the top left corner that says logical workspace and physical workspace. So i clicked it and it took me to a weird lookin inner city map. What is the purpose of the cartography maps of cities and what not?






      Thanx guys oh and is this site managed with Joomla?. I swear it is. looks just like a joomla theme..



        • 1. Re: Understanding EIGRP and packet tracer 5
          Scott Morris - CCDE/4xCCIE/2xJNCIE


          Let me start out with a couple things here...



          First, I have no earthly idea what your second question is about as I've never seen that program, so perhaps someone else may answer it!



          Second, in some cultures, those who question the details of voodoo magic are often destined for sacrifice. But that being said, let's look at your EIGRP question.



          Reported Distance or Advertised Distance (same thing) are values advertised to neighbors. So it would be the lowest current distance for a particular route.



          Feasible Distance on the other hand is the lowest "known distance" as reported to your router.



          The short answer? Your feasible distance is your neighor's reported distance. YOUR reported distance will have the metric values for your directly connected link added to it.



          Let's look at other protocols as example. RIP. If Router A has a route and reports it as metric 1 to Router B, that's Router A's reported metric and Router B's feasible metric. Now, Router B will not advertise out a metric of 1 still. It'll add to it. And thus goes the advertisements.



          Feasibility Condition is a fancy way of comparing whether you are getting better or crappier metrics than before! A route in passive state (e.g. life looks ok, so I'm not "actively" freaking out and asking people about a route) can only react to better metrics received. If I receive something with a higher metric on a route that I believe is good, there's no sense in paying attention to it. The only time I would believe a worse metric is if I've already determined that something maybe wrong with the route (hence it being in active state and my sending queries about it!).



          All routers with decent advertisements of a route are feasible successors. Personally I'd perfer this be called "Possible Successors" or something like that, but it's not. The Successor (aka Next-Hop-Elect) is the lowest distance/best path.



          For a protocol that is designed to have voodoo magic and make life easier from simple management and upkeep, EIGRP is excessively complicated on its wording!












          • 2. Re: Understanding EIGRP and packet tracer 5


            Sorry for the late reply. I was in class and i couldnt find my thread.






            I couldnt agree more. Voodoo magic is way more simplistic than EIGRP. I mean, my cuts and bruises are healing way fast now a days.



            Let me see if i have this correct. If all goes well. the image should be uploaded to this post. It is my EIGRP set up.



            Reported distance is what Router D tells C and then Router C tells A. Router B also reports a distacne to A.



            The feisible distance is where i get stuck again. Doesnt each router have its own fesible distance and reported disntace?



            Who is going to be the Succesor for the netowrk and for the network?



            What about fesible successor?



            Thanx guys











            • 3. Re: Understanding EIGRP and packet tracer 5


              Each router will calculate a distance to each network based on the metric, which in the case of eigrp I think it depends on the base speed of each link to each hop(next router) in between the router and the network. Therefore, in answering your question for Successors for each of the switched networks, it depends on which of the other networks you are trying to connect from. Successors are the best routes, these routes are added to the routing table.



              Like it was mentioned earlier, the terminology is not very friendly.



              For example if your at Router A the route (successor) would probably be A to B to with a feasible successor possibly of C to D to B. But if the A to B link fails it will choose the new route thru C to D to B, but if B goes down, there will be no route to From A to C to D to, with feasible possibly being B to D to, likewise if D fails there will be no route to



              Notice I said probably, because you can set the parameters that eigrp uses to calculate the metric on each link, thus making it choose a specific route, and also forcing it to choose a feasible successor. Each router will only look at the next hop router to calculate the FD (feasible distance), then it will choose the lowest FD to be the Successor, then it will decide if any other routes meets the feasibility condition (to avoid loops), if any do then it will add that one(s) as a feasible successor.



              The feasibility condition and the metric parameters the eigrp uses can make things more complicated. In your example their probably will not be any feasible successors, and you will probably have to set the parameters on the different links, but like I said it depends.









              • 4. Re: Understanding EIGRP and packet tracer 5

                Hi LT72884,

                Regarding packet tracer just go thru this link (pdf file) ... u 'll get an idea... http://www.cisco.com/web/learning/netacad/downloads/pdf/PacketTracer5_0_Brochure_0707.pdf ......In logical workspace of PT 5.0 u can see only a logical connection of devices (end to end).... in order to see how physically devices are connected.....go to physical workspace and click the icon "working closet" on top most right corner......u can see how ur devices are physically connected.....



                Hope it helps.....

                • 5. Re: Understanding EIGRP and packet tracer 5


                  hmm interesting. I understood it that EIGRP will base its metric off of link speed. so to get to from router A you would go to router C then to D then to B.






                  • 6. Re: Understanding EIGRP and packet tracer 5

                    If you check carefull Router A will be a FS for router B to reach Having C as successor and a RD from router A that meets the feasibily condition.