EIGRP Explained | Step by Step

Oh Matt Donald had a farm E I gr P hey what's up guys welcome to sur pros in this video we're gonna be talking about AIG RP okay so AIG RP or enhanced interior gateway routing protocol is a cisco protocol or at least it was cisco created this protocol but has since released it as an open standard that being said it is nowhere near as established or supported as OSPF by third-party vendors EW GRP is an interior gateway protocol meaning it's designed to be used within a single autonomous system it is an advanced distance vector protocol it works slightly differently from other distance vector protocol such as rip and it also includes some link states like features as well as we've all written protocols the goal of EIGRP is to learn the best route to any given subnet within the network if you've already seen the OSPF videos you will know that OSPF does this by building a map of the entire network and then choosing the best routes from that map now that's all well and good but is it really necessary well II I gr P doesn't think so er GRP sends updates from one Rooter to the next almost like a game of Chinese whispers which apparently other countries call the telephone game from the perspective of each router only their neighbors exist and frankly they don't care about anything else for example Rooter one wants to add the route to the subnet 10.0.0.0 now Ruta one only sees his three neighbors and they all have roots to the 10.0.0.0 subnet so they all send over their best roots along with the metric root of one then run some quick calculations to work out which route is best in this case ruta two wins ruta one would then advertise this winning route to any downstream neighbors and so on and so forth so that's the general idea but being the technical experts that we are we want to know exactly how this works EW GRP has a three-step process becoming neighbors to Reuters running AI GRP on the same link form a neighbor relationship exchange routing information the neighbors will then exchange topology information containing all the known routes and their metrics choose the best routes each router then chooses the best route to add to its routing table we'll start with becoming neighbors so here we have two rooters as we've OSPF EIGRP forms a neighbor relationships with their connected rooters thankfully this process is a lot less involved than it is with OSPF once a IG RP is enabled on a Reuter it will start looking for potential neighbors using a hello message hello messages are also used to maintain neighbor relationships they let the other rooters know that they're still alive hello messages are sent every five seconds by default there is also a whole timer similar to OSPF dead time the whole timer is how long a Rooter should wait without hearing a hello message before it assumes that neighbor is dead this is 15 seconds by default these hello messages are sent to the multicast address of 2 to 4 0 0 10 when a Rooter receives a hello message and run some checks to decide whether or not to become neighbors with that Rooter these checks are a lot less strict than they are with OSPF they must have the same autonomous system number or a s number this is set when configuring a igr p on the Rooter they must be on the same subnet ew gr uses K values when calculating the metric these values are must match on both Reuters but to be honest they're not often changed finally if you're using authentication this of course needs to match as well it's worth noting here that AIG RP doesn't require the hello and hold timers to match if everything matches then the Reuters will respond with a Hello message of his own and that's pretty much it we now have two neighboring Reuters the next step is to exchange routing information an important note at this stage is ERG RP doesn't actually use UDP or TCP to send update type messages instead it uses reliable transport protocol or RTP this is used to reliably send these type of messages the idea is RTP will use sequence numbers to identify if messages have been received by the neighbors TIG RP uses the diffusing update algorithm also known as duel to handle all route computations to ensure no routing loops occur so let's break this down both Reuters will send full update messages which contain all routing information known by that Rooter an AK message is then sent to acknowledge the delivery once all routing information has been shared from that point onwards only partial updates will be sent if a change occurs in the network if there are no changes then only the friendly hello message will be sent back and forth hello hello hello hello forever and ever let's say a link does go down and there are no backup routes which we will talk about in a minute the router will start something called route read computation this is where the router will try and find a loop free route to the last subnet first the route will enter an active State now this is a funny one because you would think active means working but no active means the root is active leaf if a link is working it's in the passive State ah working so the link is down and there are no backup routes in this case the connecting Rooter sends a query message to all of its neighbors asking them if they have any routes to the lost subnet the neighbors will reply with IVA a new route to the lost subnet or the tower the Reuter he's out of luck if the neighbor doesn't have a root that root is debt and it should be removed from the routing tables we now move on to our last step choosing the best routes so now we have exchanged our routing information the Reuters need to calculate the best routes and add these to their routing table hopefully everything has been pretty straightforward up until now but this is where things can start to get a little more complicated don't worry though once you get your head around this it all becomes very simple first we need to take a look at the metric calculation formula now take a deep breath 10 to the power of 7 divided by the lowest bandwidth and kilobits per second plus the delay delay is a value given to each outgoing link this is measured in microseconds multiply all of that by 256 whoo okay for example in this network ruta 1 is trying to calculate the metric to the 10.0.0.0 subnet all links are fast ethernet with a hundred thousand kilobits per second and a hundred microseconds delay note that in this calculation we use tens of microseconds so the delay we will actually be using is 10 rather than a hundred Cisco commands however will use microseconds yeah I know it's confusing it is so if we calculate this it's 10 to the power of 7 so that equals 10 million divided by our lowest link bandwidth which in this case is a hundred thousand kilobits per second because well they're all a hundred thousand kilobits per second if there was one say 10000 kilobits per second then we would use that instead plus all of our outbound delay 10 plus 10 plus 10 all of this multiplied by 256 equals our metric off 33,000 280 if you think that sounds complicated you're right it is very complicated but guess what that is the simple version yeah it gets harder I mentioned K values when we looked at the parameters that need to match when becoming neighbors and this is what's being used here band width k1 and delay k3 are all the default options when running this calculation however you can add load k2 and reliability k4 into the calculation luckily though load and reliability are rarely used it's also worth knowing that bandwidth and delay can be changed manually to influence the metric calculation now why would you want to do that well as you'll see in a minute we can adjust the metric and force the traffic to go down the route we actually wanted to okay so now for some good news while it's good to know how reuters are actually calculating these metrics it's unlikely that you will ever need to do this yourself even in the exam I can't see them asking you this question at least not this in-depth anyway and in the real world if this ever does come up for some reason then at least you can use a calculator let's move on we've seen how the metric is calculated so now we need to talk about report the distance or Rd and feasible distance or FD reported distance is the metric from a neighboring Reuters perspective reported distance can sometimes be called advertised distance as well feasible distance is the reported distance plus the distance to the neighboring Reuter who told you about the route in the first place so imagine speaking to your next-door neighbor and you say hey man you know where Jeff lives and your neighbor says Jeff yeah Jeff lives two doors down from me that is your reported distance your neighbor has reported you that Jeff lives two doors down from him so now you're like if Jeff lives two doors down from you and I live next door to you then Jeff lives three doors down from me that is your feasible distance it's the reported distance plus your distance on top hopefully that makes sense finally we need to look at how Reuters choose the best route to add to their routing table and for this we need to know what a successive root is and what a feasible successive root is a successor root is simply a root with the best metric and it will be added to the routing table ERG RP can have multiple equal cost successive roots and it will load balanced the traffic it also supports unequal load balancing with a little bit of configuration now a feasible successor is a backup root if the success of root fails having a feasible successor as a backup allows AIG RP to converge quickly with barely any downtime at all and this is a big advantage not all routes however can be feasible successors the golden rule for a feasible successor is that the reported distance must be less than the successes feasible distance why this is put in place to stop any routing loops be informed now this doesn't mean a root won't be used if the reported distance is more than the successes feasible distance nope it just means the root will need to do some recalculation just to make sure that it won't cause any routing loops now if you're not following this don't worry let's take a quick look at an example and hopefully put this all together Ruta one is trying to add the route to the 10.0.0.0 subnet the neighbors will send their best route and the route metric this is the feasible distance from the perspective of the neighboring ruta ruta wan notes this metric down as the reported distance Ruta one them runs its own calculation using the reported distance and also adding his own bandwidth and delay into the calculation the result is ruta once feasible distance it will do this for all received routes and then choose a successive route and maybe a feasible successive routes as well if we open up route to one and type show IP route a I gr P and use the pipe command B for begin 10.0.0.0 so we only see this route we can see the feasible distance in the routing table you can see that we currently have three roots to the 10000 subnet these are all successive routes because II I gr P is a load balancing the equal cost paths to see a feasible successor we will need to make the change and force one of the routes to be removed from the routing table to do this we will open Ruta - and changed the interface bandwidth enable configure terminal interface fastethernet 0/1 and we will change the bandwidth from a hundred thousand kilobits per second to ninety-nine thousand kilobits per second by typing bandwidth ninety-nine thousand now if we go back to root of one and run the same show IP route command we can now see the route through route 2 2 has been removed but the route hasn't been removed completely if we run the command show IP GRP topology this will show the EIGRP routing information this will show us the successor as well as any feasible successive routes we can also see the feasible distance on the left-hand side and the reported distance on the right we can see that we have two successor roots and the feasible distance of the successive roots is thirty three thousand two hundred and eighty a successive route is a route or routes with the best metric the bottom route has a metric of 33,000 536 which means this is not a successor route however it is a feasible successor this means the route can be used as a backup if the successor was to fail without needing to perform a recomputation that's it for EIG RP there's lots more we could talk about but I think that's enough for one video there's a few new concepts here but hopefully this is cleared up for you ERG RP is a lot simpler than OSPF however OSPF is dominating the dynamic routing world if you liked this video remember to Like comment and subscribe the support from you guys is unreal and it really does keep these videos coming thank you for watching

Channel: CertBros

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Keywords: eigrp explained, eigrp routing protocol tutorial, cisco eigrp, eigrp, cisco, enhanced interior gateway routing protocol, cisco ccna, routing protocols, how eigrp works, cisco routing and switching, advanced distance vector routing protocol, cisco training, routing and switching, distance vector, advanced distance vector

Id: QyymlFWDEgM

Length: 18min 42sec (1122 seconds)

Published: Wed Mar 20 2019