Enhanced Interior Gateway Routing Protocol

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 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.

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.