Route redistribution is one of many concepts that are very important to understand at the professional level. The Cisco ROUTE exam has a whole section (Implement an IPv4 or IPv6 Based Redistribution Solution, given a network design and a set of requirements), which is dedicated to redistribution and it is an integral part of both the CCIE written and practical exams. Many modern corporate networks utilize more than one routing protocol across their networks, and redistribution is a key piece of this configuration. Because there are so many different metric detail differences between these various protocols, it can be quite a task to really understand how redistribution works and how it can be correctly configured. This article covers the basics of redistribution (what it is for) and the configuration (how to do it) of simple redistribution between the most widely used routing protocols.
The first thing that must be said is that redistribution is really not all that confusing of a concept once the fundamentals are understood. As many modern networks use more than one routing protocol, it is quite common that they must intersect at some point. It is at this mating point where redistribution is an option. There are, however, other options, including forgoing the configuration of redistribution and utilizing static routes. This option is quite common when the number of networks is rather small and the simplicity of a static route or two is easier then figuring out redistribution configuration.
Any routing protocol supported by Cisco is able to be redistributed into any other routing protocol. There are, however, more common intersections; these will be covered in this article. These include the redistribution of RIP, OSPF, and EIGRP into each other. While BGP can be redistributed as well, it is not something to be done lightly and is not typically something the standard engineer deals with often and as such is not covered in a basic article.
Let's review how RIP, OSPF, and EIGRP utilize metrics; we will do them in order:
- RIP utilizes a simple metric which is based on the hop count.
- OSPF utilizes a metric which is based on the link speed, the specific formula is
- EIGRP utilizes a metric that can be a bit more confusing, but by default only uses link bandwidth (speed) and delay. The specific formula is
- However by default, only k1 and k3 are equal to 1 while k2, k4 and k5 are equal to 0. What this means is that only bandwidth and delay are considered so by default the formula looks like this:
So what is the point of all this in a redistribution article? Well, the understanding of how these metrics are calculated by each protocol is important when trying to select the metrics which are going to be assigned to the redistributed traffic.