In this lesson, well take a look at eigrp enhanced interior gateway routing protocol which is ciscos routing protocol. If there is an fs, it is placed in the routing table. What value represents the advertised distance of the feasible successor for the 10. A passive state in eigrp is a route which is oky and have not issue, a route for have there is an active successor route. Additional feasible successors are candidates as for unequalcost paths to be included in the routing table. So lets type the command necessaryto get in to our topology table which issh ip eigrp topology. A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. Eigrp stub routing feature improves network stability, reduce resources utilization and simplifies stub router configuration. In order for a route to become a feasible successor, the following condition must be met. Only successors and feasible successors appear when using the command show ip eigrp. If you are unfamiliar with distance vector and rip i highly recommend to read my introduction to rip first before continuing eigrp stands for enhanced interior gateway routing protocol and is a routing protocol created by cisco.
Eigrp will always install the successor with the best path. The successor route through r2, and feasible successor routes through r3 and r4. Multiple routes in an active state indicate an unstable eigrp network. How to find successor and feasible successor in eigrp. Eigrp does contribute to this table, however, and therefore it is usually involved in the discussion of eigrp. Eigrp also makes use of another concept called split horizon, which we will also briefly touch on later in this article. It uses distance information metric to provide an efficient loopfree path. Eigrp maintains redundant routes in memory so that if one fails, reverting to an backup route can occur instantaneously. Now you might be thinking that this sounds familiar.
Successor and feasible successoreigrp techexams community. Named mode does not enable eigrp for ipv4 routing unless configured. Chapter 10 eigrp eigrp overview eigrp neighbors eigrp. A feasible successor is a backup path to reach that same destination that can be used immediately if the successor route fails. Total metric as seen by a local router to reach a specific destination. This means forward all packets destined for that network to null 0 which is a blackhole and therefore will be discarded. Eigrp lab 2 eigrp fault tolerance objective in this lab eigrp will be configured over a full mesh topology and then tested to observe dual replace a successor with a feasible successor after a link failure. A successor routes may be backed up by a feasible successor route.
In this lesson, ill explain how eigrp selects a successor and feasible successor route. The feasible successors stored in the topology table are what makes it possible for eigrp to converge rapidly or even instantly. Eigrp cheat sheet successor primary route to a destination. How eigrp works with feasible succesor cisco community. Eigrp has low usage of network resources during normal operation. Successor routes are stored in both the topology table and the routing table. Eigrp is a routing protocol whose strategy to prevent routing loops is based upon what is known as the eigrp feasibility condition. Module 2 configuring eigrp lesson 1 introducing eigrp eigrp capabilities and attributes. Eigrp stub routing feature improves network stability, reduce resources.
For paths with feasible successors convergence time is in the milliseconds. Thus, the route through router b to the destination. A neighboring router that meets the feasibility condition for a. Make sure you see this feasible successor route not only in the topology table but also in the routing table. If the path is lost, the router examines the topology table to. Successor is best path in eigrp whereas feasible successor is backup path. Feasible successor a feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. Eigrp stub advertises summary and directly connected routes.
Eigrp refers selected route as successor and backup route as feasible successor. In part 4 of this eigrp video boot camp, well add a broadcast segment to the configuration and see the feasibility concept in action with successor and feasible successor routes on live. B was his successor, and he had no feasible successor so would a have responded with infinity, or would have had to have asked c, d, e first. Reported distance has to be less than successors composite metric to be eligible to become a feasible successor. The enhanced interior gateway routing protocol eigrp, referred to as an advanced distance vector protocol, offers radical improvements over igrp. If the primary route successor fails the fs is immediately installed from t. To define who will be the feasible successor, we need to consider the fd of the local router and the metric of any possible successor.
A feasible successor is a backup path used in the event the successor route disappears. Eigrp is going to keep up to six feasible successors in the topology table. Cisco proprietary protocol fast convergence loop free dual diffusing update algorithm. If available, dual and the eigrp topology database will also calculate a guaranteed loop free backup route called the feasible successor route. Designing scalable eigrp designs designing cisco network. Scenario the international travel agency wants to run eigrp on its core, branch, and regional routers. Eigrp will keep up to six feasible successors in the topology table. To figure out, we have to compare the advertised distance of that route with the feasible distance of the successor s route, if ad feasible successor. Path with lowest computed distancefd is called successor. Successor and feasible successor explained two terms that you will often encounter in the eigrp world are the successor and feasible successor. Eigrp selects single best route for each destination from this list. How many feasible successor routes will eigrp store in the topology table. That metric is called the feasible distance and is stored in r1s routing table 30720 in our case.
Lab testing by cisco has shown that the key factor for eigrp convergence is the presence or absence of a feasible successor. Both paths ace and ade will be selected as feasible successor routes. A feasible successor is a backup route, which is not stored in the routing table but stored in the topology table. Because it is not the leastcost or lowestmetric path, therefore it is not being selected as the. Router for which the successor route fails and no feasible successor route exist moves to an active state and compelling the eigrp process to send out querypackets and for reconverge. D eigrp will automatically use the route with the lowest feasible distance fd e eigrp will automatically use the route with the lowest advertised distance ad answer. B is not correct because neighbor table only contains a list of directly connected eigrp routers that have an adjacency with this router, it doesnt contain successor routes. The rule states that in order to be considered a feasible successor route, the advertised distance ad of the feasible successor should be less than the feasible distance fd of the successor the advertised distance is the distance advertised by an upstream neighboring router to a particular route destination. Icnd2 eigrp question 1 what does a router do if it has no eigrp feasible successor route to a destination network and the successor route to that destination network is in active status. The feasible and reported distance are displayed in r1s eigrp topology table. We also have a feasibility condition for a path to be feasible successor.
Nov 01, 2017 in this article ill explain the basics of eigrp to cisco ccna students. Pdf improvement of performance of eigrp network by using a. The route that is to be inserted to the routing table is selected by the dual is depended on the feasible successors. Join lazaro diaz for an indepth discussion in this video understanding eigrp, part of advanced cisco routing. Feasible successors a destination entry is moved from the topology table to the routing table when there is a feasible successor. Another two terms that appear often in the eigrp world are successor and feasible successor. When a neighbor changes the metric it has been advertising or a topology change occurs in the network, the set of feasible successors may have to be reevaluated. A destination entry is moved from the topology table to the routing table when there is a feasible successor. Best path determined by the router to reach a destination. Feasible successors are important as they allow an eigrp router to recover immediately upon network failures and hence reduce the number of dual computations and therefore increase performance. Pdf a survey of the eigrp standard and following opensource. You can verify that routers have become neighbors by using the show ip eigrp neighbors command on either router. Ensure router bates can use the framerelay network as a feasible successor route to network 4.
If the eigrp topology table lists the querying router as the successor for this route and a feasible successor does not exist, then the router queries all of its eigrp neighbors except those sent out the same interface as its former successor. Ccnp route chapter 2 lab 21, eigrp load balancing version 7. A reported distance that is not less than the feasible distance, i. Not all alternative routes to a particlar network will become feasible successors.
Sep 24, 2018 to configure eigrp for ipv6 we must explicitly specify a router id before it can start running. In this article ill explain the basics of eigrp to cisco ccna students. Introduction to enhanced interior gateway routing protocol. Eigrp feasible successor is a backup node that can satisfy the eigrp feasibility condition. A successor route is used by eigrp to forward traffic to a destination and is stored in the routing table. Like other linkstate routing protocols, eigrp uses eigrp hello packets to establish relationships with neighboring eigrp routers. When a route current successor goes down, the router first checks its topology table for a feasible successor but it cant find one. If no feasible successor exists and a link fails, a route will enter an active converging state until an alternate route is found. From this set, the neighbors that have an advertised metric less than the current routing table metric are considered feasible successors.
Eigrp interview questions and answer online networks. The feasibility condition fc states that, to be considered a feasible successor fs the advertised distance ad a. The controller inform router 1 to divide flow bc between. Routes with equal feasible distance are installed by default in the routing table. Feasible successor a destination entry is moved from the topology table to the routing table when there is a feasible successor. It can be seen in the eigrp topology table and because of this. Each router has a directly connected subnet that needs to be advertised through eigrp. In the topology table, eigrp prepares a backup successor the feasible successor.
Feasible successor fs is the best alternative loopfree backup path to reach a destination. These routes are stored in the eigrp topology table. With the exception of igrp and eigrp, each routing protocol utilizes a unique and thus incompatible metric. Eigrp uses separate topology table for each routed protocol. The protocol used to manage communication between eigrp. The network depicted above consists of only two routers. A feasible successor is a backup route that can be used in the event of a topology change without having to recalculate routes. Eigrp is a routing protocol based on distance vector technology. A table maintained by eigrp speaking router to track the information of neighbors. To figure out, we have to compare the advertised distance of that route with the feasible distance of the successors route, if ad aug 31, 2019 router for which the successor route fails and no feasible successor route exist moves to an active state and compelling the eigrp process to send out querypackets and for reconverge. If a feasible successor exists, a route should never enter an active state. All minimum cost paths to the destination form a set.
Eigrp general hybrid rputing protocol uses dual algorithm protocol independant classless routing protocol. When a link to a neighbor that is the only feasible successor goes down, all routes through that neighbor commence a route recomputation and enter the active state. The feasible successors flow distribution subalgorithm in network 1. Feasible successors provide eigrp with redundancy, without forcing routers to reconverge thus stopping the flow of traffic when a topology change occurs. Specifically, eigrp will contribute only the successor route to the routing table.
The feasibility requirement prevents the route from becoming a feasible successor. Fsisaneighborwhosreportedor advertised distance adrd is less than the current feasible distance fd for that particular route. Question what is the purpose of using protocoldependent modules in eigrp. It is backed up by a feasible successor route that. Were going to go back into our topology tableand lets talk a little bit moreabout this feasible successor. Eigrp really hates the querying process so it always preferred to have a backup route in case the link to a destination is unreachable. Path ace will be selected as a feasible successor route. The route must be loopfree, a current feasible successor in the topology table.
It does this with the concept of successors and feasible successors. An eigrp router doing manual summarization will have null 0 assigned to this summary. Reported distance rd must be less than feasible distance fd of the successor. In this case, because ad 155160 feasible successor. Any show command to show the eigrp feasible succcessor. Which type of eigrp route entry describes a feasible. A feasible successor is a path whose reported distance is less than the feasible distance current best path. Finally, the command show ip eigrp topology alllinks can be used to show the entire topology table including successor, feasible successor, and non successor routes. Feasibility condition simply means that the backup router should be loopfree. Eigrp interview questions and answer online networks solution. Jul 28, 2011 change the eigrp kvalues so only delay will be used as a metric. Lets examine the topology shown below figure1 to understand how eigrp finds loopfree alternatebackup node.
Successor selection, feasible successor, advertised and feasible distance. Ccnp route chapter 2 exam answers version 7 score 100%. Fs feasible successor fd feasible distance rdreported distance im reading up on eigrp and i dont quite understand the following. Eigrp stands for enhanced interior gateway routing protocol and is a routing protocol created by cisco. However, this is not categorized as a route recomputation. Feasible successor is a route whose advertised distance ad is less than the feasible distance fd of the current best. Eigrp successor and feasible successor a l l a b o u t n. The convergence time upon a successor failure with a feasible successor exists is in the range of 2 to 4 seconds 1 ping drop. Feasible distance, reported distance, and feasible successor. To properly discuss the feasibility condition however, you must have an understanding of the following eigrp terms. Feasible successor routes are maintained in the topology table only. Mar 26, 2017 it is backed up by a feasible successor route that is stored in the topology tableif one is available. Question when an eigrp enabled router uses a password to accept routes from other eigrp enabled routers, which mechanism is used. Notice that eigrp for ipv6 routerid must be an ipv4 address.
Fast reroute means we switch to another next hop, loopfree alternate is an alternative path in the network that is loop free. Eigrp loopfree alternate lfa fast reroute frr is a feature that allows eigrp to switch to a backup path in less than 50 ms. A successor is the route with the best metric to reach a destination. Customers have been using eigrp to achieve subsecond convergence for years. The feasibility condition of eigrp syracuse university. Eigrp successor routes will be placed into the routing table. The feasible successor can be used instantly, which means convergence time is instantaneous. In case the successor goes down, the feasible successor quickly goes into the routing table. Advertised distance cost between next hop and a destnation. Apr 27, 2019 both paths ace and ade will be selected as feasible successor routes.
C successor routes are flagged as active in the routing table d a successor route may be backed up by a feasible successor route. A route is placed in an active state when the successor and any feasible successors fail, forcing the eigrp to send out query packets and reconverge. If the rd is less than the fd the route is added in the topology table and marked as fs. Ciscos enhanced interior gateway routing protocol eigrp. To further understand how a feasible successor is selected, assume that the successor route to 192. Sep 18, 2012 in this video you will see how to configure eigrp and explain the processes what are happening on the background. Routing metrics are a key consideration when performing route redistribution. If you are not sure about eigrp, please read my eigrp tutorial question 1. Introduction to eigrpenhanced interior gateway routing protocol. If there is no feasible successor in the table, a multicast is sent out to find a new route. Originally, it was only available on cisco hardware but since a few years, its now an open standard.
1610 825 463 942 1245 1203 1506 620 690 528 1631 744 267 340 505 991 340 101 1252 1071 852 386 815 1259 898 1209 827 761 683 167 420 323 1306 503 1099