There are times that EIGRP will not install routes because of a duplicate
router ID problem. 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.
