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Enhanced Interior Gateway Routing Protocol


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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)
More Information

11. Enhanced Interior Gateway Routing Protocol

Enhanced Interior Gateway Routing Protocol, also known as either EIGRP or Enhanced IGRP, is a relatively new innovation from Cisco that is based on IGRP. EIGRP shares its predecessor's distance-vector technology, but differs greatly in its operational mechanics. Additionally, EIGRP features several important new features. Some of these features were designed to expand the market potential for EIGRP. Unlike its predecessor, for example, EIGRP supports both classless and class-based IP addresses, as well as other network protocols. This allows networks using CIDR-compliant IP addresses and/or VLSM to use Cisco's distance-vector routing technology.

Other updates were designed to reduce convergence times and improve network stability. One such update was a new algorithm, the Diffusing Update Algorithm (DUAL), which enables EIGRP routers to determine whether a path advertised by a neighbor is looped or loop free and allows a router running EIGRP to find alternative paths without waiting for updates from other routers. This helps EIGRP networks converge, without incurring any significant risk of introducing or propagating routing loops. Other measures were also introduced that reduced the network intensity of convergence. This chapter examines the architectural framework of EIGRP, its operational mechanics, and the similarities and differences between EIGRP and IGRP.

EIGRP Background

The Internet is a highly dynamic network. Consequently, its protocol, IP, must be equally dynamic. The IETF, it seems, never ceases upgrading, modifying, and extending the IP specification. One of the unfortunate consequences of this sort of activity is that many other IP-centric technologies are forced to either keep the same pace of innovation or fall into obsolescence. Frequently, such technologies opt for some point in between these two extremes. IGRP, despite its success in the market, began to demonstrate some of the effects of obsolescence relative to the ever-evolving IP.

IGRP is still a highly useful routing protocol, but it just can't support some of the more complex changes made to IPv4. IGRP is, for example, inherently a classful IP routing protocol. Therefore, it can't support classless interdomain routing (CIDR), or even variable-length subnet masks (VLSM).

Rather than modify IGRP, which may have caused migration issues for its customers, Cisco Systems decided to implement the needed modifications in a new proprietary, distance-vector routing protocol. This strategy held many benefits for Cisco's customers. For example, Cisco could offer customers a modern and robust routing protocol, EIGRP, which was based on IGRP's proven distance-vector technology.

Equally important, developing EIGRP separately from IGRP offered Cisco the opportunity to make fundamental improvements to the operational efficiency of the new routing protocol. Had Cisco attempted to make such changes to IGRP, the result may have been two very different versions of IGRP. The last benefit of Cisco's strategy was that there was no impact on any of the existing IGRP customer networks; IGRP soldiered on, unaffected by the development of EIGRP.

Backward Compatibility with IGRP

Although EIGRP was developed as a more up-to-date and efficient alternative to IGRP, it was also explicitly an extension of IGRP. Consequently, the two are designed to be completely compatible. These two routing protocols even share the same distance-vector routing technology: EIGRP uses the same composite routing metric as IGRP. EIGRP also supports all the same distance vectors, and their mathematical weights, as does IGRP. EIGRP also uses IGRP's Variance feature to provide unequal-cost load balancing.


There are strong similarities between many of IGRP and EIGRP's basic components. These include the formula for calculating their respective composite metrics, as well as equal or unequal-cost load balancing using the Variance mechanism. If you would like more information on any of these topics, refer to Chapter 10, “Interior Gateway Routing Protocol.”

There is only one minor difference in the algorithm that calculates the composite metric: The IGRP metric is 20 bits long, whereas the EIGRP metric is 32 bits long. This difference results in the EIGRP metric being larger by a factor of 256 than a comparable IGRP metric for any given route. The larger metric allows a better and finer mathematical comparison of potential routes.

This minor difference is easily and automatically compensated for by EIGRP. EIGRP automatically adjusts the composite metric of IGRP routes and adjusts its own metric on routes being redistributed to IGRP routers. IGRP's and EIGRP's metrics are directly comparable. Therefore, they can be used interchangeably after translation. EIGRP does, however, track the translated IGRP routes as external routes.


IGRP doesn't have any concept of internal and external routes. Consequently, EIGRP routes that are translated and redistributed into an IGRP network are treated as IGRP routes.

Automatic redistribution between IGRP and EIGRP will only occur if the two protocols are configured with the same autonomous system (AS) number. If they have different AS numbers, they will assume that they are part of different networks (that is, autonomous systems).

EIGRP's automatic metric adjustment mechanism enables IGRP and EIGRP to be fully integrated in a network via a simple mathematical function. External IGRP routes that are automatically adjusted by EIGRP can be directly compared to internal EIGRP routes. Cisco routers will always select the route with the best metric (adjusted or otherwise) rather than automatically select the route of any particular protocol. Therefore, an EIGRP router might decide that the best route is actually an external IGRP route rather than an internal EIGRP route.

Migration from IGRP to EIGRP can be done gradually without incurring any network downtime. EIGRP can be introduced into strategic areas of the network such as the backbone. Its automatic metric translation mechanism would enable the network administrator to replace IGRP with EIGRP in those strategic areas. Support for IGRP is integral to EIGRP so the network's functionality isn't compromised. The network administrator can then selectively extend its use in the network, until the migration to EIGRP is complete. At this point, some of the more advanced IP architectures, such as VLSM and classless addressing, can be implemented.

EIGRP's Improvements

Cisco also instituted several changes in EIGRP that were designed to improve its operational efficiency relative to IGRP. These two protocols are interoperable thanks to their mutual distance vectors, but they operate in very different ways. EIGRP reacts to topological change differently, advertises routes differently, and even has a different approach to updating entries in routing tables. In many ways, EIGRP behaves more like a link-state routing protocol than it does a traditional distance-vector protocol. Yet, EIGRP uses the distance vectors and composite metric of IGRP. Consequently, EIGRP is sometimes referred to as a hybrid routing protocol (or an advanced distance-vector protocol). It combines the best features of link-state routing with the best features of distance-vector routing. Properly designed and implemented, an EIGRP network is extremely stable and efficient and converges rapidly after any topological change.

Some of the specific advantages of EIGRP include the following:

  • Minimal consumption of bandwidth when the network is stable—During normal, stable network operation, the only EIGRP packets exchanged between EIGRP nodes are hello packets. This simple handshake enables the EIGRP routers to know that all remains well in the network.

  • Efficient use of bandwidth during convergence—EIGRP only propagates changes to the routing table, not the entire routing table. Also, updates are advertised only after a topological change rather than on a strict, periodic basis. These updates are also only transmitted to those EIGRP routers that need to know of the change. This is known as a partial, bounded update.

  • Rapid convergence—EIGRP routers store every path they have learned to every destination in the network. Therefore, a router running EIGRP can quickly converge on an alternative route after any topological change.

  • Support for VLSM and CIDR—EIGRP supports the definition of network and host numbers on any bit boundary, per interface, for both IP addresses and subnet masks.

  • Complete independence from routed protocols—EIGRP is designed to be completely independent of routed protocols. Support for routed protocols is via individual, protocol- specific modules. Therefore, evolution of a protocol, such as IP, won't threaten EIGRP with obsolescence. Nor will such technological advances force a painful revision of EIGRP.

The features highlighted in this list are examined in more detail throughout this chapter. Combined, they make a compelling case for migrating to EIGRP.


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