| EIGRP is rooted in the Cisco-compatible and adaptable nature of the distance-vector protocol IGRP. EIGRP offers several improvements and enhancements over IGRP, which make EIGRP a good choice for newer , growing network implementations . The opportunity for a reduction in bandwidth utilization is a primary benefit. EIGRP, like Open Shortest Path First (OSPF), uses multicast addresses as opposed to broadcast packets to propagate most of the operational traffic. EIGRP also has advantages that are due to its link-state behavior; for instance, EIGRP also offers route summarization at any bit position and route redistribution from other protocols. The algorithm used by EIGRP to provide a loop-free environment is called the Diffusing Update Algorithm (DUAL), which offers fast convergence. DUAL is the decision-making mechanism for determining what information will be stored in the topology table, as well as for performing all route computations . DUAL also keeps tabs on all of the advertised routes of the routers' peers, provides a loop-free circuit to every destination, and injects this information into the EIGRP routing table. In addition, EIGRP transmits incremental updates instead of the whole IP routing table.  | The EIGRP router configuration distance command is used to manually configure the administrative distance value for an internal or external EIGRP route. |
EIGRP uses IP packets to share routing information and uses protocol number 88 in its IP header. It is a very flexible routing protocol that operates over multiaccess topologies such as token ring and ethernet, as well as WAN technologies such as dedicated, point-to-point, and nonbroadcast multiaccess (NBMA) topologies. EIGRP supports hierarchical and variable-length subnet mask (VLSM) addressing, supernetting, and route summarization at major network boundaries by default. Here is a list of the most important features of Cisco's Enhanced Interior Gateway Routing Protocol: -
Bases metric on a composite of bandwidth, delay, and maximum transmission unit (MTU) sizes to offer the optimum path to a destination -
Enjoys a rapid convergence time and reduced use of bandwidth -
Sends only network changes instead of periodic changes -
Load balances up to six equal or unequal paths -
Provides support for multiple routed protocols -
Offers a loop-free, easy-to-configure environment -
Provides for VLSM, classless routing, and discontiguous networks -
Supports routing update authentication -
Is backward compatible with Cisco's IGRP protocol As with any full-featured dynamic routing protocol, there is a wide range of concepts and terminology to digest from the outset. Table 7.1 describes much of the EIGRP terms and concepts necessary to master this powerful protocol. Table 7.1. EIGRP Terms and Concepts | EIGRP Term or Concept | Description | | Neighbors | Routers that are connected and can exchange updates. | | Neighbor table | A structure maintained by each EIGRP router that stores a list of adjacencies. An EIGRP router has a neighbor table for each supported routable protocol. | | Hello | A multicast data packet that is used to discover and manage EIGRP neighbor relationships. | | Query | A data structure that is used to address neighboring routers about a network path that is lost. | | Reply | The answer to the query packet. | | ACK | An acknowledgement that is typically made up of a Hello packet without data. | | Holdtime | The amount of time that a router waits for a Hello packet before downing a neighbor relationship. | | Topology table | A structure containing all the learned routes to a destination maintained per each configured routed protocol. | | Routing table | A structure maintained for each routed protocol that is generated from the optimal routes to a destination and injected from the topology table. | | Smooth round trip time (SRTT) | The amount of time in milliseconds necessary to transmit a packet reliably to a neighbor and receive the subsequent ACK packet. | | Retransmission timeout (RTO) | The amount of time that an EIGRP router waits before retransmitting to a neighbor from the queue. | | Feasible distance | The metric to a remote network. | | Feasible successor | An EIGRP neighbor that does not represent the least-cost path and is not the router used for data forwarding. | | Successor | A neighbor EIGRP router that meets the requirements for feasibility. The successor is the route from the topology table that will be inserted into the routing table because it possesses the best attributes for the destination. | | Active | The time period when a router is querying neighbors for information regarding a network path. | | Passive | The usual operating mode for finding a route to a destination. | | Stuck in active (SIA) | A route that is abandoned and cleared when it takes too long for an EIGRP query to be answered . | |
