In a lab, R1 connects to R2, which connects to R3. R1 and R2 each have several working
interfaces, all assigned addresses in class A network 10.0.0.0. Router R3 has some working
interfaces in class A network 10.0.0.0, and others in class B network 172.16.0.0. The
engineer experiments with the auto-summary command on R2 and R3, enabling and
disabling the command in various combinations. Which of the following combinations will
result in R1 seeing a route for 172.16.0.0/16, instead of the individual subnets of class B
network 172.16.0.0?

A.
auto-summary on R2 and no auto-summary on R3

B.
auto-summary on R2 and auto-summary on R3

C.
no auto-summary on R2 and no auto-summary on R3

D.
no auto-summary on R2 and auto-summary on R3

Explanation:
R2 has interfaces only in class A network 10.0.0.0, so the auto-summary setting
has no effect. R3 has interfaces in both class A network 10.0.0.0 and class B network
172.16.0.0, so auto-summary causes R3 to summarize all subnets of
172.16.0.0/16 as a summary route when advertising to R2.