Exam 300-101 topic 2 question 575 discussion

Actual exam question from Cisco's 300-101

Question #: 575
Topic #: 2

Refer to the exhibit. EIGRP is configured with the default configuration on all routers. Autosummarization is enabled on routers R2 and R3, but it is disabled on router R1. Which two EIGRP routes will be seen in the routing table of router R3? (Choose two.)

A. 10.0.0.0/8
B. 10.10.0.0/16
C. 10.10.10.0/24
D. 172.16.0.0/16
E. 172.16.0.0/24

Show Suggested Answer

Suggested Answer: F. 172.16.10.0/24 🗳️
Explanation -
EIGRP performs an auto-summarization each time it crosses a border between two different major networks. In this case all different networks are in different major networks so EIGRP will perform auto-summarization when it exits an interface. But R1 has been configured with no auto-summary command so EIGRP will not summarize on S0 interface of R1. So the routing table of R2 will have the network 10.10.10.0/24 (not be summarized).
When exiting S1 interface of R2, EIGRP summarizes network 172.16.10.0/24 into the major 172.16.0.0/16 network but it does not summarize network
10.10.10.0/24 because it is not directly connected with this network. Therefore in the routing table of R3 there will have:
+ Network 10.10.10.0/24 ( not summarized)
+ Network 172.16.0.0/16 (summarized)
-> C and D are correct.
Note: I simulated this question on GNS3, you can see the final outputs of the show ip route commands on these routers (I connected these routers via
FastEthernet, not Serial interfaces so the outputs are slightly different but the main points are not changed).