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How does redistribution take place between EIGRP and IGRP when they belong to the same autonomous system?

Cisco How-To Tutorials
Interior Gateway Routing Protocol (IGRP), a Cisco proprietary distance vector routing protocol, is similar to the Routing Information Protocol (RIP) in behavior. However, IGRP uses a composite metric that utilizes high bandwidth paths over low bandwidth links. IGRP provides equal and unequal cost load balancing to utilize multiple paths available to reach a destination. Enhanced IGRP (EIGRP) is also a Cisco proprietary enhanced distance vector routing protocol. EIGRP relies on the Diffused Update Algorithm (DUAL) to calculate the shortest path to a destination. EIGRP is similar to IGRP in calculating the metric, but has many improvements. These include fast convergence, incremental updates and support for multiple network layer protocols. The metric for an EIGRP route is 256 times higher than the metric of an IGRP route for the same destination. When two domains using these protocols need to communicate with each other, redistribution between EIGRP and IGRP is necessary. These are the two issues that occur with EIGRP and IGRP redistribution belonging to the same autonomous system:

1. Internal EIGRP routes that (by default) have a lower administrative distance value of 90 are always preferred over external EIGRP or IGRP routes that (by default) have a value of 170 and 100.
2. External EIGRP route metrics are compared to scaled IGRP metrics. The route with the lowest metric value is used. If the metric values are the same, the external EIGRP route is preferred. However, external EIGRP can be ignored, even though their routes have a higher administrative value than IGRP.

Both IGRP and EIGRP use an Autonomous System (AS) number and only routers using the same AS number can exchange routing information using that protocol. When routing information is propagated between IGRP and EIGRP, redistribution has to be manually configured because IGRP and EIGRP use different AS numbers. However, redistribution occurs automatically when both IGRP and EIGRP use the same AS number. Because IGRP and EIGRP use compatible metrics, the metrics are scaled up or down by 256. This is when they are redistributed from one to the other. For more information refer to: Redistribution Between EIGRP and IGRP in the Same Autonomous System

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