The idea behind route redistribution sounds so simple: routing protocol A will tell routing protocol B about its known paths. For the most part, when dealing with one-way redistribution solutions, it can be this simple; the complexity of redistribution comes with multiple routing protocols, two-way redistribution points, and multiple entry and exit points. When all of these different things exist within a redistribution situation, the configuration can get very complex very quickly. This article takes a look at some of the most common problems that are seen when redistributing to and from Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP).
Things to Look for When Using EIGRP Redistribution
There are a couple different things that can be commonly misconfigured when setting up EIGRP redistribution. The following sections go over a couple of these and how they can be remedied.
Routing Table Entries
One thing that must be hammered into the mind of the configuring network engineer is that only routes that actively exist within the routing table will be redistributed. For example, if a router is being configured to redistribute OSPF into EIGRP, only those OSPF routes that are actively in the redistributing router’s routing table will be redistributed into EIGRP. This is not a behavior that is specific to EIGRP; it is the same regardless of dynamic routing protocol.
EIGRP default metric
Unlike most of the other dynamic routing protocols, EIGRP does not automatically have a default metric for any learned routes. What this means is that the redistribute command syntax can be perfectly correct, but nothing will actually be redistributed. To remedy this situation, ensure that either the metric keyword is used inside the redistribute command used under the EIGRP router configuration mode, or that the default-metric command is used.
The exception to this rule is when IGRP and EIGRP are being run on the same router and are redistributing between each other. In this case, the metrics are automatically converted between them and injected.
EIGRP Administrative Distance
Many network engineers forget that unlike other internal routing protocols, EIGRP does not use the same administrative distance for internal and externally sourced routes. “Normal” internal EIGRP routes by default use an administrative distance of 90 and are typically considered higher in priority than other common internal routing protocols (OSPF – 110, IS-IS – 115, RIP – 120). However, when a route is redistributed into EIGRP, it does not use an administrative distance of 90; it uses an administrative distance of 170. This way the router will always prefer EIGRP route sources from inside the network over externally sourced routes.
For those just learning dynamic routing protocols and redistribution, please take the time to test many different redistribution scenarios, either with live equipment or with a product like dynagen or GNS3. The further a candidate gets into Cisco’s certification path, the more complex the redistribution examples get and will be tested on. This time is very important because many engineers do not have to alter the redistribution configuration of a live production network until a change or problem occurs. Having this added experience makes the editing of these commands much easier as a ground level understanding will exist.