EIGRP uses the concept of Router ID to avoid routing loops. Each route carries the router ID of the router that injected it into EIGRP. If a router see as route with its own Router ID then it knows a routing loop has occcured.
To see the Router ID for a specific network, use the following command:
R1#show ip eigrp topology 2.2.2.0/24
or R1#show eigrp address-family ipv4 topology 2.2.2.0/24
EIGRP-IPv4 VR(test) Topology Entry for AS(100)/ID(1.2.1.1) for 2.2.2.0/24
<snip>
Originating router is 1.1.1.1
Alternatively, as of IOS version 15.0(1)M you can use the following commands:
R1#show ip protocols <snip> Router-ID: 1.1.1.1 <snip> R1#show eigrp protocols <snip> Router-ID: 1.1.1.1 <snip>
The Router ID in this case is 1.1.1.1. To change it use the following commands (in named mode).
R2#show run | sec eigrp
router eigrp test
!
address-family ipv4 unicast autonomous-system 100
!
topology base
exit-af-topology
network 0.0.0.0
eigrp router-id 1.1.1.1
exit-address-family
As per OSPF a configured router ID is preferred, otherwise the highest local “up” loopback IP is used. If there is no loopback configured, then the highest “up” interface is used. The router ID remains the same until the EIGRP process is restarted or the router ID is manually configured or removed. The values of 0.0.0.0 and 255.255.255.255 are not allowed. You need to be carefull that two routers do not have the same router ID, as they will reject each others routes, when they see a route with their own router ID installed.
There is no warning of this except for an obsecure message you can find in “show eigrp address-family ipv4 events” command:
R1#show eigrp address-family ipv4 events Event information for AS 100: 1 15:17:24.455 Ignored route, metric: 2.2.2.0/24 metric(1392640) 2 15:17:24.455 Ignored route, dup routerid int: 1.1.1.1
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