Four routers run IS-IS and have established adjacency relationships. The area numbers and router levels are marked in the figure. What is the Cost value for R4 to reach 10.0.2.2/32?

Question

Options

• A10
• B20
• C30
• D40

• A
  3% (1)
• B
  6% (2)
• C
  82% (28)
• D
  9% (3)

Explanation

Option C (30) is correct because IS-IS assigns a default link cost of 10 per interface, and R4 must traverse 3 links to reach 10.0.2.2/32 (typically a loopback on R2), giving a total cost of 10 + 10 + 10 = 30. Option A (10) is wrong because it would imply R4 is only one hop away from the destination, which contradicts the multi-router topology shown. Option B (20) is wrong because it accounts for only two links, missing at least one intermediate hop in the path R4 takes through the IS-IS domain. Option D (40) is wrong because it overstates the path by one extra link - the topology does not require four hops to reach that prefix.

Memory tip: In IS-IS, the default narrow metric cost is 10 per link - simply count the number of links along the shortest path and multiply by 10. Think of it as "IS-IS costs a dime a hop."

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