EIGRP includes many new technologies, each of which represents an improvement in operating efficiency, speed of convergence, or functionality relative to IGRP and other routing protocols. These technologies fall into one of the following four categories:

• Neighbor discovery and recovery
• Reliable Transport Protocol
• DUAL finite-state machine algorithm
• Protocol-dependent modules

Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors, much the same way that OSPF routers do.

EIGRP routers establish adjacencies as described in Figure . EIGRP routers establish adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable or passive. By forming adjacencies, EIGRP routers do the following:

• Dynamically learn of new routes that join their network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable

Reliable Transport Protocol (RTP) is a transport-layer protocol that can guarantee ordered delivery of EIGRP packets to all neighbors. On an IP network, hosts use TCP to sequence packets and ensure their timely delivery. However, EIGRP is protocol-independent. This means it does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do. To stay independent of IP, EIGRP uses RTP as its own proprietary transport-layer protocol to guarantee delivery of routing information.

EIGRP can call on RTP to provide reliable or unreliable service as the situation warrants. For example, hello packets do not require the overhead of reliable delivery because they are frequent and should be kept small. Nevertheless, the reliable delivery of other routing information can actually speed convergence, because EIGRP routers are not waiting for a timer to expire before they retransmit.

With RTP, EIGRP can multicast and unicast to different peers simultaneously, which allows for maximum efficiency.

The centerpiece of EIGRP is the Diffusing Update Algorithm (DUAL), which is the EIGRP route-calculation engine. The full name of this technology is DUAL finite-state machine (FSM). An FSM is an algorithm machine, not a mechanical device with moving parts. FSMs define a set of possible states that something can go through, what events cause those states, and what events result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm will react to a set of input events. The DUAL FSM contains all the logic used to calculate and compare routes in an EIGRP network.

DUAL tracks all the routes advertised by neighbors. Composite metrics of each route are used to compare them. DUAL also guarantees that each path is loop free. DUAL inserts lowest cost paths into the routing table. These primary routes are known as successor routes. A copy of the successor routes is also placed in the topology table.

EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL selects alternate routes quickly by using the information in these tables. If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table.

One of the best features of EIGRP is its modular design. Modular, layered designs prove to be the most scalable and adaptable. Support for routed protocols, such as IP, IPX, and AppleTalk, is included in EIGRP through PDMs. In theory, EIGRP can easily adapt to new or revised routed protocols, such as IPv6, by adding protocol-dependent modules.

Each PDM is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following:

• Sending and receiving EIGRP packets that bear IP data
• Notifying DUAL of new IP routing information that is received
• Maintaining the results of DUAL routing decisions in the IP routing table
• Redistributing routing information that was learned by other IP-capable routing protocols