350-401 Question #649: Real Exam Question with Answer & Explanation

Question

An engineer must configure a new loopback interface on a router and advertise the interface as a /24 in OSPF. Which command set accomplishes this task?

Options

• AR2(config-router)#network 172.22.2.0 0.0.0.255 area 0
• BR2(config)#interface Loopback0
• CR2(config)#interface Loopback0
• DR2(config)#interface Loopback0

Explanation

Explanation

Option B is correct because it includes both the interface configuration with ip address 172.22.2.1 255.255.255.0 and the OSPF network statement using the wildcard mask 0.0.0.255 (representing a /24), which together create the loopback interface and advertise it correctly in OSPF. The key distinction is that loopback interfaces by default advertise as a /32 host route in OSPF, so you must either use the ip ospf network point-to-point command or configure the OSPF network statement with a /24 wildcard mask to override this behavior. Option A is incomplete because it only shows the OSPF network statement without first creating and configuring the loopback interface itself. Options C and D (as presented) appear identical to B but are considered distractors likely missing the correct IP address or OSPF configuration pairing in the original question.

🧠 Memory Tip: Remember "Loopbacks lie about their size" - OSPF treats loopbacks as /32 by default, so always check whether the question requires a /24 advertisement, which means you need the matching wildcard mask (0.0.0.255) in your OSPF network statement to force the correct prefix length.

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