I. EIGRP Concept  EIGRP uses the Diffused Update Algorithm (DUAL) algorithm to calculate the shortest path to the target network. EIGRP is a private routing protocol invented by Cisco, developed from IGRP, but the algorithm has been greatly changed. EIGRP is the same as IGRP and RIP. A dynamic routing protocol using DV algorithm. The algorithm of EIGRP has been greatly changed.Although it is a dynamic routing protocol that uses DV algorithm like IGRP and RIP, it has greatly improved the convergence speed, occupied network bandwidth and system resources, and has fast convergence For loopless route calculation, the DUAL update algorithm EIGRP can ensure that 100% does not form a loop. The convergence speed makes it impossible for the EIGRP protocol to generate loop routes in route calculation, and the convergence time of route calculation is also well guaranteed.  Because the DUAL algorithm allows EIGRP to calculate the route, it will only recalculate the changed route; for a route, only the router affected by this route will be involved in the route recalculation. During normal operation, the network resource utilization rate is very low; only Hello packets are transmitted on a stable network. EIGRP can also control the EIGRP packets sent, reducing the occupation rate of EIGRP packets on the interface bandwidth, thereby avoiding the continuous sending of a large number of routing packets Things that affect the normal data business occur.  EIGRP (Enhanced Interior Gateway Routing Protocol) employs critical metrics such as AD (Administrative Distance), FD (Feasible Distance), FC (Feasibility Condition), and FS (Feasible Successor) to optimize routing decisions. In EIGRP, AD represents the trustworthiness of a route, and lower values indicate higher reliability. The routing table lists routes learned from multiple routing protocols, each with its AD. To modify the administrative distance for a specific protocol, like EIGRP, use commands such as "r1 config router distance." FD, on the other hand, measures the total cost of reaching a destination, computed by routers. EIGRP uses this metric to select the best path to a destination network. FC is crucial in EIGRP's topology database, where it ensures that only routes satisfying the condition (AD < FDmin) are stored as feasible successors. To view EIGRP routes and their metrics, employ the "show ip route command." In situations where redundancy is essential, a floating static route can provide backup options. Routers like R1 can thus make efficient routing decisions based on these metrics for a resilient network infrastructure. II. What is eigrp dual algorithm  Although EIGRP is a distance vector routing protocol, after receiving a route from a neighbor, it is not directly used in the routing table without any calculation. EIGRP will put all the routes received from the neighbor into the topology database (Topology Database ), The optimal route is put into the routing table after calculation by DUAL's acyclic algorithm; because EIGRP may have multiple neighbors or may receive the same route from multiple neighbors, it is necessary to select the optimal route from Routes are put into the routing table for use instead of the optimal route. The route is backed up in the topology database. After the route in the routing table fails, the alternate route is searched from the topology database and placed in the routing table. After EIGRP puts the routing information received from neighbors into the topology database, it needs to select the optimal route through the DUAL algorithm. DUAL is a convergence algorithm, which replaces the Belloman-ford algorithm used for other distance vector protocols. The design idea of the DUAL algorithm is that even a temporary routing loop will damage the performance of a network, so in order to break the routing loop at any time, a distributed shortest path routing is performed using diffusion calculation.  The DUAL algorithm has several important terms--AD / FD / FC / FS    III. AD value  For the router E, the metric value of the B\C \ D routers reaching the Z network respectively is called the AD value of E. That is, the distance from the neighbor to the destination network segment (advertised distance), such as the E-B-Z path, for the E router, its AD value is 10.  IV. FD value  For router E, the total metric of the path reached by E-B-Z, called the FD value of E, is calculated from the local router to the sum of the metrics between the target networks. For example, the path of E-B-Z, for the E router, its FD value is : 20 (E-B) + 10 (B-Z) = 30; there may be multiple paths to the destination in the EIGRP topology database, and the optimal one put in the routing table is called the easy distance (FD)  V. Successor router:  The successor is a directly connected neighbor router, through which it has the shortest route to the destination. That is, the next-hop router with FD min In the three paths of E-B-Z, E-C-Z, and E-D-Z, C is the successor router for E, because the FD of E-C-Z is 20 Feasible successor router (FS) value: Because there may be multiple paths in the topology database to reach the destination, but the one selected as the optimal FD is put into the routing table, and the alternate route left in the topology database is called Easy Successor (FS ), Choose FS must meet FC conditions.  VI. FC value:  There can be up to 6 FSs in the topology database. If an EIGRP has 8 neighbors that can go to the destination, after selecting an FD and putting it into the routing table, not all the other 7 can be stored in the topology database. The topological database can only have a maximum of 6 (including FD), and it is not necessarily that 6 will be put into the topological database, because to be stored in the topological database, it must meet certain conditions, called Feasibility Condition ( FC), if AD <FDmin is satisfied, it is considered feasible, and the path of this neighbor can exist in the topology database. Take the above diagram as an example, FDmin = 20, and the FC condition must be satisfied, that is, AD <20. For the three paths of E-B-Z, E-C-Z, and E-D-Z, the AD of E-B-Z is 10, the AD of E-C-Z is 10, and the AD of ED-Z is 25, so only the two paths of E-B-Z and E-C-Z satisfy the FC condition, that is, only the two of E-B-Z and E-C-Z The path will be selected into the topology database.  VII. View the routing table:  R11#sh ip route eigrp Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP a - application route + - replicated route, % - next hop override Gateway of last resort is not set 145.12.0.0/32 is subnetted, 1 subnets D        145.12.12.12 [90/1603] via 145.67.89.2, 00:04:42, Ethernet0/0 145.67.0.0/16 is variably subnetted, 5 subnets, 2 masks D        145.67.89.16/30 [90/1203] via 145.67.89.2, 00:04:42, Ethernet0/0 For the route 145.12.12.12, its default AD value is 90 and FD value is 1603.  VIII. R11#sh ip eigrp topology  EIGRP-IPv4 Topology Table for AS(145)/ID(145.11.11.11) Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, s - sia Status P 145.67.89.16/30, 1 successors, FD is 1203 via 145.67.89.2 (1203/1103), Ethernet0/0 P 145.12.12.12/32, 1 successors, FD is 1603 via 145.67.89.2 (1603/501), Ethernet0/0 View routes that meet FC conditions in the topology database