EIGRP, Feasible Distance

everybody this is J Mack from Raider dogs calm today we're going to be talking about feasible and reported distance in EIGRP now for some reason this always seems to be a very complicated issue with a lot of my CCNA students and it could be because I get tongue-tied when I talk about it or could just be the crazy way that Cisco has named this kind of stuff so hopefully this will kind of clear it up for you all right now feasible distance is the lowest cost path from your router to the destination so in other words it's what the local router see so router one sees a path to router five his best path would be the feasible distance all right now report a distance reported distance is what the neighboring routers refer to as your feasible distance so in other words after you find your feasible distance to r5 you would then report that to other routers so your feasible distance gets reported to the other routers so reported distance is the feasible distance of other routers and feasible distance is your best distance to the destination site so really it's the exact same thing but it depends on how you're looking at it so for example here my son might call me Dad but as friends call me Jay Mac so on the same person but I get called different names depending on the perspective and it does it gets kind of confusing so let's take a look at that from the different perspectives all right here it this is our one and our - and they're both trying to get to the destination here are five I through some different path costs on here so from R 1 to R 5 his obviously his best path is a distance of 5 a distance of 5 so his total distance is 10 so his best route to R 5 has a distance of 10 so that becomes his feasible distance so from R 1 he says hey my best path is straight to R 3 to R 5 it has a feasible distance of 10 now our - from his perspective he looks at it if he goes to our six - our four to our five then he's got three five which is eight plus five so he has a feasible distance of 13 but if he goes from our two and goes to our 1 our three here he has a feasible distance of 14 so in his case his feasible distance is 13 but when he sends that to r1 r1 records it as the reported distance of r2 does that help but also really our 2 has a feasible distance of 13 our 1 has a feasible distance of 10 so they both have a feasible distance they both have a best route or best cost to get to r5 but when r2 sends that to r1 r1 records that as the reported distance now why is it different because there's a condition later on that kind of meat takes both of these two different numbers and puts them together and that's why they have to kind of be named differently if that helps it all so just make sure at this point you understand that from your router or from the local router or how everyone look at it the best path to the destination is the feasible distance and anybody else's feasible distance that gets told to you is referred to as reported distance a reported distance is always what's reported from a neighboring router and feasible distance is always your best path your best cost now at this point it's not too bad understand but now here's where it gets weird the feasible distance is actually assign to a successor route a successor route is your best path to a destination and the successor route would show up in the routing table but the reported distance is assigned to the feasible successor and a feasible successor would show up in the topology table kind of as a backup route so here's what a little topology table would look like just this one of the network destinations oh and we're going to explain this by the different line types here in just a second but this point just make sure you're good with the feasible distance is assigned to the successor out and reported distance are assigned to feasible successors and moving that and getting worse so the successor is the lowest cost route to a destination and that route again gets placed in the routing table the feasible successor is a backup route or secondary route that meets the feasibility condition and we'll talk about that just a second but the feasible successor is not in the routing table he gets placed in the topology table remember AIG RP has three tables it has a routing table with a successor routes the best path to the destinations it has a topology table which is the backup paths to destinations and then it has a neighbor table which we'll talk about in a different video so successors show up in the routing table feasible successors show up in the topology table alright let's talk about that feasibility condition again the feasibility condition says in order to be a feasible successor the route must have a reported distance that is less than the feasible distance so here's why they named them something different we can't just say that the reported distance is actually our twos feasible distance because then this condition it would be really hard to map this out but we can say if we call it if everything gets it gets reported we call a reported distance then we can make this condition work so in order to be a backup route and be listed in the topology table the reported distance has to be less than my feasible distance so your first question is why does there have to be a condition why don't we have all the possible backup routes from one point from our network you know in our router well the reason is one it would create routing loops which are bad second thing is it would also fill up our routers with so many secondary routes that are kind of worthless and it would take a whole lot of more processing power memory things like this so it would slow our network down considerably so between routing loops and the slowness factor that you would get we don't want that we only want good backup routes all right so let's talk about that topology table again now here is one entry from the topology table so in order to get to this network 1 7 to 1700 30 it says there's one successor so if it says there's one successor but there's two entries the second entry must be the feasible successor so your topology table will also list the successor routes as the first route and they'll show it'll tell you the feasible distance is blah blah blah 4 5 6 0 and then here for the primary route he's got 4 5 6 0 so in order to be a feasible successor his reported distance the second number must be lower than the feasible distance so in this case he's 3 5 6 0 which is less than 4 5 6 0 so he does become a secondary route or a feasible successor so in your topology table you have a bunch of destination networks you'll have at least one successor because there has to be a path to get there and then if there's other entries they may be feasible successors or they may be equal cost load balancing paths or something like that so you want to look if this case had just had two entries and said two successors we would have load balancing but it doesn't it says one successor so here's the the first one is usually the successor and then everything else underneath that has to be a feasible successor if it helps alright so what happens if a route does not meet the feasibility condition let's look at that all right back to our topology table so from R 2 to R 5 he has a reported distance of 13 but from R 1 to R 5 we have a feasible distance of 10 so the feasible organ we're looking at it from our ones perspective the feasible distance is 10 but the reported distance is 13 this means that R 2 will not the path from R 2 to R sixth our Ford R 5 will not show up in the topology table and it will not become a feasible success sir now that does not mean that r1 will never use that path if for some reason this link here goes down or this link here goes down and r1 no longer has a path to our five this way what will happen is r1 will then send queries out and say hey I don't have a path to r5 who does well r2 would respond and say hey I have a path and has a distance of 13 well at that point our ones got to make a decision I either can't get there where I can get there through a distance of 13 so he's going to take the distance of 13 so that path would show up them so just because a route doesn't show up as a feasible successor does not mean your router won't use it it just means your router won't list it as an immediate backup path but if there is a problem and he needs it he will request that and he will get it and then he will use it if that makes sense but again don't forget it's always a matter of perspective so when you look at these things you have to remember where you're looking at them from in this first example we just talked about we looked at it from our ones point of view but if I change this and now we look at it from our twos point of view our twos best path is a has a feasible distance of 13 and he goes to our six to our 4 to our 5 but our 2 has a reported distance of 10 because he can get there there there but our two can't go r1 r3 r5 because then his distance would be 14 so in our twos case he can either go right or he can go left if he goes right he has a distance of 13 which is lower so that becomes his feasible distance of 13 but r1 has a reported distance of 10 because 10 is lower than the feasible distance of 13 the path to r1 would show up in the topology table for r2 and then if for some reason this path here between r2 + r6 broke our 2 would immediately go to his topology table find the backup path list of the feasible successor and then he would start sending data this way you know without having to send queries and get information back from all of his neighbors so that's why the topology table is in there so that we can make decisions very fast and have a faster convergence if not then I have to send queries I have to wait replies that come back there's always there's a stuck inactive issue that we'll talk about in another video but it can take a lot longer it can take three to four minutes to converge my network if a link breaks if I don't have those routes in my topology table but if from the top of the table the stuff happens pretty fast and most of the time users won't even notice the difference so again it's all a matter of perspective from our ones perspective our two would not show his path would not show up in the topology table but from R to R once path would because he meets the feasibility condition his reported distance is less than the R - feasible distance all right so theoretically if our network design right that's how everything would work but remember I also mentioned that the topology table in the feasibility condition helps us to prevent routing loops let me show you a quick example of that all right don't make fun of my drawing but you get the idea so we're on our one and we're sending to our five now our twos report a distance no matter what it's always going to be higher than our ones feasible distance because he has to add this in addition to this and this and our threes distance is always going to be higher than our ones because he has to add this path to this to this so our ones always going to have a faster path to our five then R 2 and R 3 so R 2 and R 3 would never want to be kept as a backup route because they would just create a routing loop if R 1 to try to get to R 5 would send to R 2 then R 2 would send what back to R 1 and then you get this route list is keep going back and forth you don't want that or if you sent to our three and then R 3 would send R 2 and R 2 it's NR 1 again you get this running loop so that's why the feasibility condition is there 1 to speed convergence up and have some backup pass - to prevent routing loops like this from occurring and we do have stuff like this in our networks because we want redundancy and we do have redundant links the case that link goes down you know we don't our network doesn't go out so that's the big thing about the feasibility condition and the feasible successors and successors prevent routing loops and to improve our convergence speed all right one more point I want to make the usability condition states that in order to be a feasible successor you have to have a reported distance less than the feasible distance okay so your reported distance has to be less than the feasible distance so when you look at these topologies and this is what they try to do to trip you up on the CCNA they want to check your understanding so here's two different networks that we can get to in our topology table they both say one successor but the CCNA will ask you which one of these would not show up in the topology table so if we look the feasible distant or the feasible distance here is listed as two zero five one four five six zero so if I go to the reported distance now remember the first one is the successor so that's the feasible distance showing up there okay the second number the second entry his second number over here is his reported distance so the reported distance of the feasible successor has to be lower than the feasible distance so in this case he ends in three five six zero three five six zero which is more than four five six zero so this reported distance is less than this feasible distance or this feasible distance how everyone look at so this first one would show up in the topology table but if we look at this second entry the feasible distance is three zero seven two zero so here's your successor route with his distance of three zero seven two zero and then this feasible successor shows up is three zero seven two zero that is not less than the feasible distance it is the same as the feasible distance and that doesn't work so it does not meet the feasibility condition so this second route here would not show up in the topology table because his reported distance here is not less than the feasible distance so don't let that one trip you up just make sure you understand that feasibility condition in order to be a feasible success or backup path your path must have a reported distance less than the feasible distance all right hope that helps and I hope that explains it to you again I've got a a detailed guide on rider dogs comm so rider dogs comm is our CCNA CCNP blog that we're just starting many times the some of our students have some issues we're gonna try to post some stuff up here a guide about the topic some tips and tricks video guides as well just like this one here and information about the coming CCNA changes cisco is reworking all of their certifications they're starting now with CCNA and that will go live they at this point they plan on having the whole the book series done by the end of 2013 so by January 2014 that's their target date they should theoretically be able to provide a new CCNA exam now some of the updates and stuff that are coming up like ip6 having a greater role and multi area OSPF and again we'll start talking about these throughout the rest of the year and get some information on you on the site so again stop by router dogs calm and if you've got some certain CCNA or cisco topic that you want us to talk about or maybe make clear for you just send us an email and we'll get it up there for you alright guys good luck and happy routing

Channel: Johnny Mac

Views: 17,381

Keywords: eigrp, feasible distance, feasible successor, successor route, backup routes, topology table, cisco eigrp, cisco feasible, successor routes

Id: 0dNyXU7Njj0

Length: 16min 43sec (1003 seconds)

Published: Tue Mar 12 2013