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New Features Found in EIGRP


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Enhanced Interior Gateway Routing Protocol



EIGRP Background



New Features Found in EIGRP



EIGRP Data Structures



The Routing Table



The Topology Table

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IP Routing Fundamentals

From: IP Routing Fundamentals
Author: Mark Sportack
Publisher: Cisco Press (53)
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New Features Found in EIGRP

EIGRP enjoys many new technologies, each of which represents an improvement in operating efficiency, rapidity of convergence, or feature/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

  • Protocol-specific modules

In addition to these categories of technological improvements, EIGRP was also treated to some other substantial enhancements relative to IGRP. These enhancements may defy easy categorization, but were needed to keep pace with the ongoing evolution of IP as well as other routing protocols. EIGRP enhances the security of a network, for example, by permitting authentication to be used between routers. This function is very similar to the authentication supported by OSPF and can be used in conjunction with any routed protocol. Other innovations such as support for VLSM and CIDR are specific only to IP. As such, they are integrated in a protocol-specific module rather than included in EIGRP itself.

Neighbor Discovery and Recovery

EIGRP, unlike virtually every other distance-vector routing protocol, does not rely exclusively and rigidly on the use of timers for maintaining its routing table. Instead, the basis for maintaining routing tables is a periodic communication between EIGRP routers. They use this process to

  • Dynamically learn of new routers that may join their network

  • Identify routers that become either unreachable or inoperable

  • Rediscover routers that had previously been unreachable

The basic neighbor discovery/recovery process consists of periodically transmitting a small hello packet to neighbors. The hello packet establishes the relationship between immediate neighbors (known as an adjacency). This relationship is used to exchange routing metrics and information.

An EIGRP router can safely assume that, as long as it is receiving hello packets from known neighbors, those neighbors (and their routes) remain viable. If an EIGRP router ceases to receive such greetings from a neighbor, however, it can assume that something is amiss. That router will enter the DUAL process for those routes. These processes are examined in more detail in the section titled “Hello Packets.”

Reliable Transport Protocol

One of EIGRP's more important new features is its capability to provide guaranteed, reliable delivery of its various packets. Other protocols eschew reliable delivery and rely on other mechanisms such as the passing of time to determine whether a packet needs to be retransmitted. Unfortunately, the fundamental flaw in such an approach is that it aggravates the convergence process. The longer it takes a network to converge, the greater the opportunity for disrupting service across the network. EIGRP was given a new protocol, the Reliable Transport Protocol (RTP), to provide reliable delivery of its own packets.

RTP is a transport layer protocol that correlates to the functionality identified by Layer 4 of the OSI reference model. RTP is a private innovation of Cisco Systems, however, and is not an open standard. IP uses two similar transport protocols: TCP and UDP. In fact, RTP embodies some of the functionality of each TCP and UDP. TCP—as was explained in Chapter 5, “Internet Protocols Versions,”—provides reliable delivery of IP datagrams and can resequence datagrams received out of order. UDP provides a more efficient, but unreliable, delivery of IP datagrams. RTP can support both reliable and unreliable, delivery of datagrams. It can even support both transport types simultaneously and resequence packets received out of order.

Instead of creating a new transport protocol, the designers of EIGRP could have used TCP and/or UDP as the transports for EIGRP messages. However, this would have made EIGRP distinctly IP specific. The designers' goal was a truly protocol- independent routing protocol that could easily be extended to support any new routing protocols, such as IPv6, that may be developed in the future. Consequently, a new transport protocol was needed: RTP. RTP was specifically developed to meet these requirements.

RTP is used to transport all EIGRP message types through a network. However, not every EIGRP packet requires reliable delivery! Some functions, such as the exchanging of hello packets, just don't warrant the overhead of acknowledging receipt. RTP can deliver hello packets (and other packet types) in an unreliable manner.

RTP can also support both multicasting and unicasting. Multicast packets are delivered to multiple, specific destinations simultaneously using a group address. Unicast packets are explicitly addressed to a single destination. RTP can even support both multicast and unicast transmissions simultaneously for different peers.

The Distributed Update Algorithm

The centerpiece of EIGRP's new technologies is DUAL, EIGRP's route-calculation engine. The full name of EIGRP's engine is DUAL Finite-State Machine. This engine contains all the logic used to calculate and compare routes in an EIGRP network. DUAL tracks all the routes advertised by neighbors and uses the composite metric of each route to compare them. Selected paths must be loop-free paths and have the lowest cost. Such routes are inserted by the DUAL protocol into a routing table for use in forwarding datagrams.

Routes selected for insertion in a routing table are also evaluated on the basis of feasible succession. A feasible successor is a neighbor router that is the next hop in a least-cost path to any given destination. A feasible successor is a path that is loop-free according to the DUAL FSM.

Protocol-Specific Modules

As indicated earlier in the chapter, one of the key design principles guiding the development of EIGRP was complete independence from routed protocols. Therefore, EIGRP implemented a modular approach to supporting routed protocols. Many other protocols are either specifically designed for a single routed protocol—such as IP, AppleTalk, and so on—or have mechanisms for supporting multiple protocols. EIGRP has such native mechanisms, but they are completely modular. In theory, EIGRP can be easily retrofitted to support any new routed protocols that may be developed by just adding another protocol-specific module.

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

  • Sending and receiving EIGRP packets that bear IP data

  • Notifying DUAL of new IP routing information that is received

  • Maintaining the results of DUAL's routing decisions in the IP routing table

  • Redistributing routing information that was learned by other IP-capable routing protocols

IP-EIGRP can redistribute routes learned from other IP-capable routing protocols, including OSPF, RIP, Integrated Intermediate System-Intermediate System (IS-IS), Exterior Gateway Protocol (EGP), and Border Gateway Protocol (BGP). EIGRP has comparable modules for supporting both AppleTalk and IPX. The AppleTalk module (AT-EIGRP) can redistribute routes learned from the Routing Table Maintenance Protocol (RTMP). IPX-EIGRP can redistribute routing information from Novell's proprietary version of RIP as well as that company's Service Advertisement Protocol (SAP) and Novell Link State Protocol (NLSP).

EIGRP's IP-EIGRP module brought support for many of the updates to IP that Cisco's IGRP customer base had been clamoring for. Specifically, IP-EIGRP introduced support for VLSM as well as CIDR. IGRP did not support either of these features.


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