Forming adjacencies in EIGRP with primary addresses on different subnets

In EIGRP it is best to only use the network statement for primary addresses of an interface, if adjacencies are required to form properly on that link - the primary address, meaning the address configured on an interface without the secondary added to the end of it.

What you can do in EIGRP is, that you can enable it to run using the network command that matches a secondary address of an interface, but that will only make the process run on that interface - the process will use the primary interface to form adjacencies with any peers. Below is a the configuration of two routers, router R1 and R2. They are connected through a switch on interface GigabitEthernet1.

Router R1 configuration

R1(config)#interface GigabitEthernet1
R1(config-if)#ip address 10.0.0.1 255.255.255.0
R1(config-if)#ip address 172.16.0.1 255.255.255.0 secondary
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#router eigrp 1
R1(config-router)#no auto-summary
R1(config-router)#network 10.0.0.1 0.0.0.0

Router R2 configuration

R2(config)#interface GigabitEthernet1
R2(config-if)#ip address 172.16.0.2 255.255.255.0
R2(config-if)#ip address 10.0.0.2 255.255.255.0 secondary
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#router eigrp 1
R2(config-router)#no auto-summary
R2(config-router)#network 10.0.0.2 0.0.0.0

The output of a show ip eigrp neighbors command will on both routers shows some confusing information.

EIGRP adjacency table on R1

EIGRP adjacency table on R2

R1 shows a neighbor of 172.16.0.2 on interface Gi1 and R2 shows a neighbor of 10.0.0.1 on interface Gi1.

The addresses that form the neighborship are the primary addresses on the interfaces, where EIGRP is enabled. On R1 the primary address is 10.0.0.1 and on R2 the primary address is 172.16.0.2. The primary address is the address that is used for EIGRP messages regardless of which secondary address is used to enable the EIGRP process on an interface.

I want to point out here, that I actually expected some error messages to pop up and adjacencies to fail between the two routers, but it appears that these routers running IOS 15.4 (they're actually CSR1000v routers) are more capable than the routers I ran this type of scenario on back when I was studying for my CCNP ROUTE exam.

Nonetheless, this type of configuration will most likely not work on routers running older versions of IOS and in the case here, where the adjacencies do form, it gives a somewhat confusing output from the show ip eigrp neighbors command and also in the next-hop addresses of the routing tables.

So, in short, be sure to use the same common subnet as primary addresses for EIGRP enabled interfaces - it may work, but at best it gives confusing output when the neighbor that forms is on a subnet that is not defined in a network command under the router process.