Refer to the exhibit. After recent configuration changes to a customer's network, a network engineer notices that R2 cannot communicate with R3. Both FastEthernet interfaces on R2 and R3 are up and…

Question

Exhibit

Options

• AConfigure virtual links between R1 and R3.
• BConfigure interface F0/0 on R1 and R2 to be in area 0.
• CDefine area 2 as a NSSA on R2 and R3.
• DConfigure a loopback interface on R2 and assign it to area 0.

• A
  4% (1)
• B
  11% (3)
• C
  7% (2)
• D
  78% (21)

Explanation

Option D is correct because OSPF requires every ABR (Area Border Router) to have at least one interface connected to Area 0 (the backbone). If R2 is acting as the ABR between Area 0 and Area 2 (where R3 resides) but has no interface in Area 0, it cannot perform inter-area routing - adding a loopback assigned to Area 0 is the minimal change that gives R2 backbone membership and restores reachability to R3.

Why the distractors are wrong:

• A (Virtual links): Virtual links are a workaround for a discontiguous Area 0 and must be configured between two ABRs, not an end router like R3 - this doesn't solve the problem here.
• B (Move F0/0 on R1 and R2 to Area 0): Changing an interface already involved in adjacencies would break existing OSPF neighbor relationships, particularly R2's link to R3 in Area 2, making things worse rather than better.
• C (NSSA): A Not-So-Stubby Area is only needed when an area has an ASBR injecting external routes while also needing stub-like filtering - there's no ASBR scenario described here.

Memory tip: Remember the OSPF golden rule - "all roads lead through Area 0." If an ABR is missing its backbone connection, a loopback interface is the surgeon's scalpel: it adds Area 0 membership without disturbing any existing interfaces or adjacencies.

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