In the final section of this chapter we are going to be looking at how to troubleshoot basic route redistribution issues when using EIGRP. In order to effectively troubleshoot redistribution issues, you must have a solid understanding of how redistribution into EIGRP works. By default, when external routing information is redistributed into EIGRP, the external routes are assigned a default metric of infinity. The only three exceptions to this rule are:
- When redistributing between two EIGRP autonomous systems
- When redistributing static routes into EIGRP
- When redistributing connected interfaces (subnets) into EIGRP
EIGRP preserves all metrics when redistributing between the two EIGRP autonomous systems. When connected subnets (interfaces) and static routes are redistributed into EIGRP, the redistributed routes are assigned a default external metric value of 0. When redistributing any other external route source, such as from OSPF for example, a metric must be manually specified for those external routes. In Cisco IOS software, this can be performed using one of three methods. These three methods are:
- Specifying the seed (default) EIGRP metric for redistributed routes
- Indirectly specifying the EIGRP metric during redistribution using a route map
- Directly specifying the EIGRP metric during route redistribution
The seed metric is the metric value that will be assigned to all the redistributed routes. In other words, this is the initial metric that will be assigned to the external routing information when redistribution into EIGRP is configured. The seed metric is configured using the default-metricrouter configuration command. Within a route-map, the set metric command can be used to specify the redistribution metric that will be used for the matched subnet(s). And finally, you can also specify the route metric during redistribution using the redistribute <protocol> metricconfiguration command.
Following redistribution, all EIGRP external routes are assigned an administrative distance of 170 and are printed as D EX routes in the routing table. If you have configured redistribution into EIGRP and do not see any external routes, check the following while troubleshooting:
- Verify that the routes are not incorrectly filtered
- Verify that the metric has been specified
When redistributing, ensure that filters that are used during redistribution, e.g. route maps, are configured in the correct manner and all networks that should be redistributed are permitted in the configurations. Because redistribution configurations can quickly become very complex, especially when redistributing at multiple points of the network, it is very important to check and then double-check your configuration prior to and following implementation. And finally, do not forget that EIGRP requires a metric to be specified when redistributing any routes other than EIGRP, static and connected into EIGRP.