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| 5.1 |  | EIGRP Fundamentals |
| 5.1.1 |  | EIGRP and IGRP compatibility |

Cisco released EIGRP in 1994 as a scalable, improved version of its proprietary distance vector routing protocol, IGRP. IGRP and EIGRP are compatible with each other, although EIGRP offers multiprotocol support and IGRP does not.

Despite being compatible with IGRP, EIGRP uses a different metric calculation and hop-count limitation. EIGRP scales the IGRP metric by a factor of 256. That is because EIGRP uses a metric that is 32-bits long, and IGRP uses a 24-bit metric. By multiplying or dividing by 256, EIGRP can easily exchange information with IGRP.

EIGRP also imposes a maximum hop limit of 224, which is slightly less than the 255 limit for IGRP. However, this is more than enough to support most of the largest internetworks.

Getting dissimilar routing protocols, such as OSPF and RIP, to share information requires advanced configuration. However, sharing or redistribution, is automatic between IGRP and EIGRP as long as both processes use the same autonomous system (AS) number. In Figure , RTB automatically redistributes routes learned by EIGRP to the IGRP AS, and vice versa.

EIGRP will tag routes learned from IGRP, or any outside source, as external because they did not originate from EIGRP routers. On the other hand, IGRP cannot differentiate between internal and external routes. Notice that in the

show ip route command output, EIGRP routes are flagged with D, and external routes are denoted by EX. RTA identifies the difference between the network learned via EIGRP (172.16.0.0) and the network redistributed from IGRP (192.168.1.0). RTCs table shows that IGRP makes no such distinction.

RTC, which is running IGRP only, sees only IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.

| Interactive Media Activity Drag and Drop: IGRP and EIGRP Comparison Upon completion of this activity, the student will be able to identify the differences between IGRP and EIGRP.

| Interactive Media Activity Drag and Drop: EIGRP Metric Calculation Upon completion of this activity, the student will be able to identify the equation for calculating EIGRP and IGRP metrics.

| Web Links Enhanced Interior Gateway Routing Protocol http://www.cisco.com/warp/public/103/eigrp-toc.htm