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

Content 3.1 EIGRP Concepts 3.1.5 EIGRP data structure Like OSPF, EIGRP relies on different types of packets to maintain its various tables and establish complex relationships with neighbor routers. The five EIGRP packet types are:

Hello
Acknowledgment
Update
Query
Reply

EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers. Rediscovery occurs if EIGRP routers do not receive hellos from each other for a hold time interval but then re-establish communication. EIGRP routers send hellos at a fixed but configurable interval, called the hello interval. The default hello interval depends on the bandwidth of the interface. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10. An EIGRP router stores information about neighbors in the neighbor table. The neighbor table includes the Sequence Number (Seq No) field to record the number of the last received EIGRP packet that each neighbor sent. The neighbor table also includes a Hold Time field which records the time the last packet was received. Packets should be received within the Hold Time interval period to maintain a Passive state. The Passive state is a reachable and operational status. If a neighbor is not heard from for the duration of the hold time, EIGRP considers that neighbor down, and DUAL must step in to re-evaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired. OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each of the other respective timers via the exchange of hello packets. Then they use that information to forge a stable relationship regardless of unlike timers. Hello packets are always sent unreliably. This means that no acknowledgment is transmitted. An EIGRP router uses acknowledgment packets to indicate receipt of any EIGRP packet during a reliable exchange. Reliable Transport Protocol (RTP) can provide reliable communication between EIGRP hosts. To be reliable, a sender's message must be acknowledged by the recipient. Acknowledgment packets, which are hello packets without data, are used for this purpose. Unlike multicast hellos, acknowledgment packets are unicast. Acknowledgments can be made by attaching them to other kinds of EIGRP packets, such as reply packets. Update packets are used when a router discovers a new neighbor. An EIGRP router sends unicast update packets to that new neighbor so that it can add to its topology table. More than one update packet may be needed to convey all the topology information to the newly discovered neighbor. Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors, which alerts them to the change. All update packets are sent reliably. An EIGRP router uses query packets whenever it needs specific information from one or all of its neighbors. A reply packet is used to respond to a query. If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the Active state. A query is then multicasted to all neighbors in an attempt to locate a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.

Content 3.1 EIGRP Concepts 3.1.6 EIGRP algorithm The sophisticated DUAL algorithm results in the exceptionally fast convergence of EIGRP. To better understand convergence with DUAL, consider the example in Figure . Each router has constructed a topology table that contains information about how to route to destination Network A. Each topology table identifies the following:

The routing protocol or EIGRP
The lowest cost of the route, which is called Feasible Distance (FD)
The cost of the route as advertised by the neighboring router, which is called Reported Distance (RD)

The Topology heading identifies the preferred primary route, called the successor route (Successor), and, where identified, the backup route, called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor. The EIGRP network will follow a sequence of actions to bring about convergence between the routers, which currently have the following topology information: Router C has one successor route by way of Router B. Router C has one feasible successor route by way of Router D. Router D has one successor route by way of Router B. Router D has no feasible successor route. Router E has one successor route by way of Router D. Router E has no feasible successor. The feasible successor route selection rules are specified in Figure . The following example demonstrates how each router in the topology will carry out the feasible successor selection rules when the route from Router D to Router B goes down: In