Exam 400-101 topic 1 question 108 discussion

Actual exam question from Cisco's 400-101

Question #: 108
Topic #: 1

Refer to the exhibit.

R3 prefers the path through R1 to reach host 10.1.1.1.
Which option describes the reason for this behavior?

A. The OSPF reference bandwidth is too small to account for the higher speed links through R2.
B. The default OSPF cost through R1 is less than the cost through R2.
C. The default OSPF cost through R1 is more than the cost through R2.
D. The link between R2 and R1 is congested. "Pass Any Exam. Any Time." - www.actualtests.com 122

Show Suggested Answer

Suggested Answer: A 🗳️
The default formula to calculate OSPF bandwidth is BW = Bandwidth Reference / interface bandwidth [bps] = 10^8 / / interface bandwidth [bps]
BW of the R1-R3 link = 10^8 / 100Mbps = 10^8 / 10^8 = 1
BW of the R2-R3 link = 10^8 / 1Gbps = 10^8 / 10^9 = 1 (round up)
Therefore OSPF considers the two above links have the same Bandwidth -> R3 will go to 10.1.1.1 via the R1-R3 link. The solution here is to increase the
Bandwidth Reference to a higher value using the "auto-cost reference-bandwidth" command under OSPF router mode. For example:

Router(config)#router ospf 1 -
Router(config-router)#auto-cost reference-bandwidth 10000
This will increase the reference bandwidth to 10000 Mbps which increases the BW of the R2-R3 link to 10^10 / 10^8 = 100.

by [deleted] at April 3, 2025, 6:12 p.m.

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