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| 5.6 |  | Troubleshooting EIGRP |
| 5.6.6 |  | Duplicate router IDs |

There are times that EIGRP will not install routes because of a duplicate router ID problem. EIGRP does not use router ID as extensively as OSPF. EIGRP uses the notion of router ID only for external routes to prevent loops. EIGRP chooses the router ID based on the highest IP address of the loopback interfaces on the router. If the router does not have any loopback interface, the highest active IP address of all the interfaces is chosen as the router ID for EIGRP. Figure shows the network setup for this example on EIGRP router IDs. Figure shows the pertinent configurations for the cause of this problem.

Router X is redistributing a route of 150.150.0.0/16 from OSPF into EIGRP and is sending the route several hops to Router C. Router C receives the route and sends the route as EIGRP external routes to Router B. Router B installs the route in the routing table and sends it to Router A. The debug output verifies how Router B sends the route to Router A.

Debugs and Verification
The problem is that Router A is not installing the 150.150.0.0/16 route in the routing table. As a matter of fact, Router A is not showing the 150.150.0.0/16 route in its topology table. Going back to Router B, the route is in the routing table, and the topology table appears.

Router B shows that it is getting the routes from Router C. By looking at the external data section, it can be noticed that the originating router is 192.168.1.1, which is seven hops away. The original protocol that originated the route 150.150.0.0/16 is OSPF with the metric of 64. Notice that the originating router is 192.168.1.1. Looking back at the configuration of Router A, notice that Router A also has an IP address of 192.168.1.1 configured on Ethernet 0, and it is the highest IP address on the router.

All of this evidence points to a duplicate router ID problem in EIGRP that causes Router A not to install routes. Because Router X and Router A have the same router ID (192.168.1.1), when Router A receives the route from Router B, it looks at the external data section of the route to see who is the originating router. In this case, Router A sees the originating router as192.168.1.1, which is its own router ID.

Router A does not put the route in its topology table because it thinks that it is the originator of the route and that by receiving the route back from other neighbors, it must be a loop. So, to prevent a routing loop, Router A does not put the route of 150.150.0.0/16 in the topology table. Consequently, the route does not appear in the routing table.

Router A will not install any external routes that originate from Router X because external routes carry the router ID in their EIGRP update packet. Router A will install internal EIGRP routes from Router X without any problem. The duplicate router ID problem happens only for external routes.

Solution
The solution to the duplicate router ID problem is to change the IP address of the loopback interface of Router X or to change the IP address of Ethernet 0 on Router A. The rule of thumb is to never configure the same IP address on two places in the network. Figure shows the change the loopback IP address of Router X to 192.168.9.1/32 to fix this problem. The result of the IP address change in Router X is the installment of the 150.150.0.0/16 route in Router A.