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EIGRP Concept / Comparing EIGRP and IGRP

EIGRP Concept 
Comparing EIGRP and IGRP
3.1.1

Cisco released EIGRP in 1994 as a scalable and improved version of its proprietary distance vector routing protocol, IGRP. This page will explain how EIGRP and IGRP compare to each other. The distance vector technology and distance information found in IGRP is also used in EIGRP.
EIGRP has improved convergence properties and operates more efficiently over IGRP. This allows a network to have improved architecture as well as retain the current investment in IGRP.
The comparisons between EIGRP and IGRP fall into the following major categories:
  • Compatibility mode
  • Metric calculation
  • Hop count
  • Automatic protocol redistribution
  • Route tagging
IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important as users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not.
EIGRP and IGRP use different metric calculations. EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric. EIGRP can multiply or divide by 256 to easily exchange information with IGRP.
IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support large, properly designed internetworks.
To enable dissimilar routing protocols such as OSPF and RIP to share information requires advanced configuration. Redistribution, or route sharing, is automatic between IGRP and EIGRP as long as both processes use the same AS number. In Figure , RTB automatically redistributes routes learned from EIGRP to the IGRP AS, and vice versa.
EIGRP tags routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. 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 172.16.0.0 network, which was learned through EIGRP, and the 192.168.1.0 network that was redistributed from IGRP. In the RTC table, the IGRP protocol makes no such distinction. RTC, which uses IGRP only, just sees IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.
The Interactive Media Activity will help students recognize the characteristics of IGRP and EIGRP.
The next page will explain EIGRP in greater detail.

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