CISCO CCNA 200-301 Q188

Enhanced Interior Gateway Routing Protocol (EIGRP) uses which algorithm to select the best path to the destination?

A. Diffusing Update Algorithm (DUAL)
B. Dijkstra algorithm
C. Bellman-Ford algorithm
D. Shortest Path First (SPF) algorithm

Correct Answer: A

EIGRP uses the Diffusing Update Algorithm (DUAL) to select the best path to the destination. EIGRP is a classless protocol that allows the use of variable length subnet masks (VLSM), and supports classless interdomain routing (CIDR) for the allocation of IP addresses.

EIGRP is characterized by these components:

DUAL: EIGRP implements DUAL to select paths free of routing loops. DUAL selects the best path and the second best path to the destination. The terminology used in DUAL is as follows:
– Successor: Best path selected by DUAL.
– Feasible successor: Second best path selected by DUAL. This is a backup route stored in the topology table.
– Feasible distance: The lowest calculated metric of a path to destination.

Protocol-dependent modules: Different modules are used by EIGRP to independently support Internet Protocol (IP), Internetwork Packet Exchange (IPX), and AppleTalk routed protocols. These modules act as a logical interface between DUAL and routing protocols.

Neighbor discovery and recovery: Neighbors are discovered and information about neighbors is maintained by EIGRP. A hello packet is multicast on every five seconds and the router builds a table with the information. EIGRP also enables proper operation over a Non-Broadcast Multiple Access (NBMA) point- to-multipoint network. EIGRP multicasts a hello packet every 60 seconds on the multipoint Wide Area Network (WAN) interfaces (X.25, frame relay, or Asynchronous Transfer Mode).

Reliable Transport Protocol (RTP): RTP is used by EIGRP to manage EIGRP packets. Reliable and ordered delivery of route updates is ensured using RTP.
EIGRP updates about routes can contain five metrics: minimum bandwidth, delay, load, reliability, and maximum transmission unit (MTU). Of these five metrics, by default, only minimum bandwidth and delay are used to compute the best path.

The Dijkstra algorithm and Shortest Path First (SPF) algorithm are used by the Open Shortest Path First (OSPF) routing protocol for selecting the best path to the destination, not by EIGRP.
The Bellman-Ford algorithm is used by Routing Information Protocol