6.1 EIGRP Concepts
6.1.1 Backward compatibility with IGRP
Lab Activity
  In this lab, you will learn how to enable EIGRP routing protocol.

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 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 IGRP. EIGRP also uses the IGRP variance feature to provide unequal-cost load balancing.

Note: There are strong similarities between many of the basic components of IGRP and EIGRP, including the formula for calculating their respective composite metrics, as well as equal or unequal-cost load balancing using the variance mechanism.

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 and EIGRP metrics are directly comparable; therefore, they can be used interchangeably after translation. EIGRP does, however, track the translated IGRP routes as external routes.

Note: 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 native IGRP routes.

Automatic redistribution between IGRP and EIGRP will occur only 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, ASs).

Lab Activity
  In this lab, you will learn how to configure EIGRP and IGRP routing protocols. IGRP will automatically redistribute into EIGRP if given the same AS#.

Migration from IGRP to EIGRP can be done gradually without incurring 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 network 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.