speeds. Interactive Media Activity Checkbox: Reliable Transport Protocol

Upon completion of this activity, the student will be able to identify the operation of RTP.
Content 2.1 EIGRP Fundamentals and Features 2.1.5 EIGRP Neighbor Discovery and Recovery EIGRP’s neighbor discovery and recovery enables routers to dynamically learn about other routers on their directly attached networks. It enables EIGRP routers to build their neighbor table, discover routes, and choose the best routes. Figures through illustrate how two neighbors initially discover and exchange routing information. EIGRP routers must then maintain their routing tables by constantly monitoring their links to discover new routes, or when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets . As long as a router receives hello packets from a neighboring EIGRP router, it assumes that the neighbor is functioning and the two can exchange routing information. Interactive Media Activity Drag and Drop: EIGRP Operation

Upon completion of this activity, the student will be able to identify the four key steps of EIGRP operation.
Content 2.1 EIGRP Fundamentals and Features 2.1.6 DUAL Finite-State Machine DUAL uses distance information, known as a metric or cost, to select efficient, loop-free paths. The lowest cost route is calculated by adding the cost between the next-hop router and the destination—referred to as the advertised distance (AD)—to the cost between the local router and the next-hop router. The sum of these costs is called the feasible distance (FD). A successor, also called a current successor, is a neighboring router that has a least cost path to a destination (the lowest FD) that is guaranteed not to be part of a routing loop. Successors are used for forwarding packets. Every destination for which one or more feasible successors exists is recorded in a topology table. For every destination listed in the topology table, the route with the lowest metric is chosen and placed in the routing table. Multiple successors can exist if they have the same FD. By default, four successors can be added to the routing table. However, the router can be configured to accept up to six per destination. DUAL also keeps backup paths to each destination. The next-hop router for a backup path is called the feasible successor. To qualify as a feasible successor, a next-hop router must have an AD less than the FD of the current successor route. If the route via the successor becomes invalid (because of a topology change) or if a neighbor changes the metric, DUAL checks for feasible successors to the destination route. If one is found, DUAL uses it, which avoids recomputing the route. If a suitable feasible successor does not exist, the route must be recomputed to determine the new successor. Although recomputation is not processor-intensive, it does affect convergence time, so it is advantageous to avoid unnecessary recomputations.
Content 2.1 EIGRP Fundamentals and Features 2.1.7 DUAL Example Assume that the routers in Figure have all converged. The text boxes summarize the topology table of routers C, D, and E. The numbers between the routers represent metric values. Note
The numbers between the routers do not reflect actual metric values. Simple values are being used for demonstration purposes only. Each router has at least one valid route to network (a) with an advertised distance (AD) that is less than the calculated feasible distance (FD). This route is identified as the successor route. For example: Router C also has another route to network (a) with an AD less than the FD. The route via router D has been identified as a feasible successor route because it has an AD of 2, which is still less than the FD of 3. Note
No other routes qualify as feasible successors, because all other routes on the routers have AD values that are either equal to or greater than their lowest FD. Figures through illustrate how DUAL progresses through a topology change: Figure : Assume that router D has lost connectivity to router A. Since it has no feasible successor, router D must use DUAL to calculate a new route to router A. It begins by removing the route to router B from its topology table. Figure : Router D then transitions to Active mode and forwards queries to router E and router C looking for an alternate path to network (a).

When router E receives the query from its successor, router D, it also removes the successor route from its topology table. Since it has no other successor router to network (a) it must also transition to Active mode like router D did.

When router C receives the query from router D, it removes the entry for the route to router D (the feasible successor) from its topology table. However, it does not transition to Active mode since it still has the successor route to network (a) in it table. Figure : Router C replies to the query from router D because it has a valid path to network (a) via router B. Since router E is now in Active mode, it forwards a query to router C looking for an alternate route to network (a). Figure : Router D adds the route advertised by router C to its topology table. In the meantime, router C replies to the query sent by router E. Figure : Router D then identifies the route via router C as its successor. Router E now adds the route advertised via router C as its successor and advertises it to router D. Figure : When router D updates its topology table with the information provided by router E, it will then have another router to network (a) with an AD lower than its FD. It will therefore identify the route via router E as its feasible successor route.
Content 2.2 EIGRP Components and Operation 2.2.1 EIGRP Tables DUAL selects alternate routes based on the tables kept by EIGRP. By building these tables, every EIGRP router can track all the routing information in an autonomous system, not just the best routes. EIGRP uses the neighbor table to list adjacent routers. The topology table lists all the learned routes to each destination, while the routing table contains the best route to each destination. The best route in the routing table is called the successor route. A feasible successor route is a backup route to a destination, which is kept in the topology table. Figure displays how the route information is added during the initial exchange of routing information between two EIGRP neighbors. Figure displays sample information for each table maintained by router C.
Content 2.2 EIGRP Components and Operation 2.2.2 EIGRP Neighbor Table When a router discovers and forms an adjacency with a new neighbor, it records the neighbor’s address and the interface through which it can be reached in the neighbor table. One neighbor table exists for each PDM. The EIGRP neighbor table is comparable to the adjacencies database that link-state routing protocols use in that it ensures bidirectional communication between each of the directly connected neighbors. When a neighbor sends a hello packet, it advertises a hold time, which is the amount of time that a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, the hold time expires, and DUAL is informed of the topology change. The entry in the neighbor table also includes information that RTP requires, such as round-trip timers. Sequence numbers are used