Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 204 The EIGRP Topology Table Once EIGRP neighbors form adjacencies, they will begin to share routing information. Each router’s update contains a list of all routes known by that router, and the respective metrics for those routes. All such routes are added to an EIGRP router’s topology table. The route with the lowest metric to each network will become the Feasible Distance (FD). The Feasible Distance for each network will be installed into the routing table. The Feasible Distance is derived from the Advertised Distance of the router sending the update, and the local router’s metric to the advertising router. Confused? Consider the following example: Router A has three separate paths to the Destination Network, either through Router B, C, or D. If we add up the metrics to form a “distance” (the metrics are greatly simplified in this example), we can determine the following: • Router B’s Feasible Distance to the Destination Network is 8. • Router C’s Feasible Distance to the Destination Network is 23. • Router D’s Feasible Distance to the Destination Network is 9. Router B sends an update to Router A, it will provide an Advertised Distance of 8 to the Destination Network. Router C will provide an AD of 23, and D will provide an AD of 9. Router A calculates the total distance to the Destination network by adding the AD of the advertising router, with its own distance to reach that advertising router. For example, Router A’s metric to Router B is 8; thus, the total distance will be 16 to reach the Destination Network through Router B. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 205 The EIGRP Topology Table (continued) Remember, however, that Router A’s Feasible Distance must be the route with the lowest metric. If we add the Advertised Distance with the local metric between each router, we would see that: • The route through Router B has a distance of 16 to the destination • The route through Router C has a distance of 27 to the destination • The route through Router D has a distance of 11 to the destination Thus, the route through Router D (metric of 11) would become the Feasible Distance for Router A, and is added to the routing table as the best route. This route is identified as the Successor. To allow convergence to occur quickly if a link fails, EIGRP includes backup routes in the topology table called Feasible Successors (FS). A route will only become a Successor if its Advertised Distance is less than the current Feasible Distance. This is known as a Feasible Condition (FC). For example, we determined that Router A’s Feasible Distance to the destination is 11, through Router D. Router C’s Advertised Distance is 23, and thus would not become a Feasible Successor, as it has a higher metric than Router A’s current Feasible Distance. Routes that are not Feasible Successors become route Possibilities. Router B’s Advertised Distance is 8, which is less than Router A’s current Feasible Distance. Thus, the route through Router B to the Destination Network would become a Feasible Successor. Feasible Successors provide EIGRP with redundancy, without forcing routers to re-converge (thus stopping the flow of traffic) when a topology change occurs. If no Feasible Successor exists and a link fails, a route will enter an Active (converging) state until an alternate route is found. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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