EIGRP - network command with no wildcard mask

When configuring EIGRP on a Cisco IOS router, the syntax used for the network command is important in how the router interprets the command. Advertising the network of an interface will cause the router to share the network of that interface with neighbors.

Issuing the network command without any wildcard mask will cause the router to revert to a classful wildcard mask. For example, issuing the following network command:

R1(config)#router eigrp 1
R1(config-router)#network 172.16.2.0

will result in the following in the running configuration:

R1#show run | section eigrp
router eigrp 1
 network 172.16.0.0
R1#

First of all, because the 172.16.2.0 address is a class B address, it will use a wildcard mask of 0.0.255.255. This does not appear, but it is assumed. Also notice that even though the address 172.16.2.0 was not the network address, the command appears in the running configuration as 172.16.0.0 which is the network address assuming a wildcard mask of 0.0.255.255.

Similarly, notice what happens when we issue the network command for a class A address:

R1(config)#router eigrp 1
R1(config-router)#network 10.10.10.0

will result in the following in the running configuration:

R1#show run | section eigrp
router eigrp 1
 network 10.0.0.0
R1#

The command reverts to the network address of 10.0.0.0 with an assumed wildcard mask of 0.0.0.255 which is used for a class A address.