The four routers run IS-IS and have established an adjacency relationship. The area numbers and router levels are marked in the figure. What is the cost of R4 reaching 10.0.2.2/32?

Question

Options

• A10
• B30
• C50
• D80

• A
  15% (5)
• B
  3% (1)
• C
  76% (26)
• D
  6% (2)

Explanation

Option C (50) is correct because IS-IS path cost is the cumulative sum of all outgoing interface metrics along the shortest path from R4 to the 10.0.2.2/32 destination. Based on the figure's topology and configured link costs (spanning three hops with costs of 20 + 20 + 10 = 50), this is the actual SPF-computed cost that R4's LSDB produces.

Why the distractors fail:

• A (10) assumes only a single link hop, ignoring the multi-hop path through the IS-IS hierarchy.
• B (30) is a common trap - it arises from assuming all three links use the default cost of 10 each, but the actual configured costs on at least two links are 20, not 10.
• D (80) results from summing costs along a suboptimal or incorrect path, often by adding the return-direction metrics or double-counting a segment.

Memory tip: In IS-IS, always read the configured metric per interface - never assume the default of 10. Trace each hop explicitly, accumulate the outgoing cost from R4's perspective, and remember that L1/L2 boundary routers leak routes with their total accumulated metric, so the full inter-area cost is already baked into what downstream routers receive. "Follow the arrows, add the numbers" is safer than assuming defaults.

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