As shown in the figure, a sham link is established between PE1 and PE2 through the Loopback0 interface. CE2 can learn the route of the 192.168.1.0/24 network segment (direct route on CE1) through…

Question

Options

• A4
• B10
• C20
• D30

• A
  81% (29)
• B
  3% (1)
• C
  6% (2)
• D
  11% (4)

Explanation

Option A (4) is correct because, with all default OSPF parameters, the cost from CE2 to 192.168.1.0/24 via the sham link is the sum of four interface costs: CE2-to-PE2 (cost 1) + the sham link PE2-to-PE1 (default cost 1) + PE1-to-CE1 (cost 1) + CE1's interface to the 192.168.1.0/24 network (cost 1), totaling 4. Option B (10) is wrong because 10 is a common default OSPF metric for certain link types, not the actual calculated cost along this four-hop path. Option C (20) is wrong because 20 is the default cost for OSPF external Type 2 routes; routes learned through the sham link are treated as intra-area, not external, so this metric does not apply. Option D (30) is wrong because that would represent the higher-cost backdoor link path, which OSPF deprioritizes in favor of the lower-cost sham link path.

Memory tip: Count the hops across the sham link path (CE2 → PE2 → PE1 → CE1 → destination network) - with four segments each carrying the default OSPF interface cost of 1, the total is always 4 when all parameters are left at their defaults.

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