What could be the cause of the problem?

Refer to the exhibit.

EIGRP has been configured on routers R1 and R2. However, R1 does not
show R2 as a neighbor and does not accept routing updates from R2. What could be the cause of
the problem?

A.
The no auto-summary command has not been issued under the EIGRP process on both
routers.

B.
Interface E0 on router R1 has not been configured with a secondary IP address of 10.1.2.1/24.

C.
EIGRP cannot exchange routing updates with a neighbor’s router interface that is configured
with two IP addresses.

D.
EIGRP cannot form neighbor relationship and exchange routing updates with a secondary
address.

Explanation:
EIGRP updates always use the primary IP address of the outgoing interface as the source
address. In this case R2 will use the 10.1.2.2/24 address, which is not in the same subnet of R1,
to send EIGRP update to R1. Therefore R1 does not accept this update and generates the “not on
common subnet” error message.
Answer D is a bit unclear. It should state that “EIGRP cannot form neighbor relationship and
exchange routing updates if the two primary addresses on two routers are not in the same
subnet”.
Notice that although R1 does not accept R2 as its EIGRP neighbors but R2 accepts R1 as its

EIGRP neighbor accepts R1 hello packets..
For more information about this problem, please read
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080093f09.s
html.