6.1 EIGRP Fundamentals 
6.1.1 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 IGRP's 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, slightly less than IGRP's generous 255, but more than enough to support today's 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 EIGRP-learned routes 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 for the routers in Figure , 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 that was redistributed from IGRP (192.168.1.0). In RTC's table, we see that IGRP makes no such distinction.

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