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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.
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
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.![]() 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.
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