The use of a routing protocol to advertise routes that are learned by some other means, such as by another routing protocol, static routes, or directly connected routes, is called redistribution. While running a single routing protocol throughout your entire IP internetwork is desirable, multi-protocol routing is common for a number of reasons, such as company mergers, multiple departments managed by multiple network administrators, and multi-vendor environments . Running different routing protocols is often part of a network design. In any case, having a multiple protocol environment makes redistribution a necessity.
Differences in routing protocol characteristics, such as metrics, administrative distance, classful and classless capabilities can affect redistribution. Consideration must be given to these differences for redistribution to succeed.
Distance Vector Protocols
When you redistribute one protocol into another, remember that the metrics of each protocol play an important role. Each protocol uses different metrics. For example, the Routing Information Protocol (RIP) metric is based on hop count, but Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP) use a composite metric based on bandwidth, delay, reliability, load, and maximum transmission unit (MTU), where bandwidth and delay are the only parameters used by default. When routes are redistributed, you must define a metric that is understandable to the receiving protocol. There are two methods to define metrics when redistributing routes.
The redistribution of IGRP/ EIGRP into another IGRP/ EIGRP process does not require any metric conversion, so there is no need to define metrics or use the default-metric command during redistribution. A redistributed static route takes precedence over the summary route because the static route has an administrative distance of 1 whereas EIGRP summary route has an administrative distance of 5. This happens when a static route is redistributed with the use of redistribute static command under the EIGRP process and the EIGRP process has a default route.
This output below shows a RIP router redistributing static, IGRP, EIGRP, OSPF, and IS-IS routes.
redistribute igrp 10
redistribute eigrp 10
redistribute ospf 10
redistribute isis default-metric 10
The RIP metric is composed of hop count, and the maximum value is 15. Anything above 15 is considered infinite; as an example you can use 16 to describe an infinite metric in RIP. When redistributing a protocol into RIP, Cisco recommends that you use a low metric, such as 1. A high metric, such as 10, limits RIP even further. If you define a metric of 10 for redistributed routes, these routes can only be advertised to routers up to 5 hops away, at which point the metric (hop count) exceeds 15.
Link State Protocols
It is possible to run more than one OSPF process on the same router. However, running more than one process of the same protocol is rarely needed, and consumes router’s memory and CPU cycles. You do not need to define metric or use the default-metric command when redistributing one OSPF process into another.
This output shows an IS-IS router redistributing static, RIP, IGRP, EIGRP, and OSPF routes.
redistribute rip metric 20
redistribute igrp 1 metric 20
redistribute eigrp 1 metric 20
redistribute ospf 1 metric 20
The IS -IS metric must be between 1 and 63. There is no default-metric option in IS-IS— you should define a metric for each protocol, as shown in the example above. If no metric is specified for the routes being redistributed into IS-IS, a metric value of 0 is used by default.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2764-2766). . Kindle Edition.