300-420 Question #4: Real Exam Question with Answer & Explanation

Question

A company is using OSPF between its HQ location and a branch office. HQ is assigned area 0 and the branch office is assigned area 1. The company purchases a second branch office, but due to circuit delays to HQ, it decides to connect the new branch office to the creating branch office as a temporary measure. The new branch office is assigned area 2. Which OSPF configuration enables all three locations to exchange routes?

Options

• AThe existing branch office must be configured as a stub area
• BA virtual link must be configured between the new branch office and HQ
• CA sham link must be configured between the new branch office and HQ
• DThe new branch office must be configured as a stub area

Explanation

{"question_number": 2, "question_summary": "OSPF area 2 connected only to area 1 (not area 0): how to enable full route exchange?", "correct_answer": "B", "explanation": "OSPF requires all non-backbone areas to have a direct physical or logical connection to area 0. Area 2 is connected only to area 1, creating a discontiguous backbone - a fundamental OSPF violation. A virtual link resolves this by creating a logical tunnel through area 1 (the transit area) between the ABR at the area 1/2 boundary and the ABR at the area 0/1 boundary, effectively extending the backbone to area 2. A sham link (option C) is used only in MPLS VPN environments to influence OSPF metric calculations over a backdoor link. Configuring stub areas (options A and D) would restrict LSA flooding but would not fix the disconnected backbone problem.", "generated_by": "claude-sonnet", "llm_judge_score": 4}

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