Refer to the exhibit. A network engineer applied configuration on R1 to summarize all ISIS routes, but R2 is still receiving specific routes from R1. The engineer has confirmed that both routers are c

Question

Exhibit

Options

• AR1(config-router)#summary-address 1.0.0.0 255.0.0.0 level-1-2
• BR1(config-router)#no summary-address 1.0.0.0 255.0.0.0 level-1
• CR1(config-router)#summary-address 1.0.0.0 255.0.0.0 level-1
• DR1(config-router)#summary-address 1.0.0.0 255.0.0.0 level-2

• A
  8% (4)
• B
  4% (2)
• C
  75% (40)
• D
  13% (7)

Explanation

Option C is correct because the question specifically asks to summarize routes within Level 1, and the level-1 keyword scopes the summary address to IS-IS Level 1 advertisements only - this tells R1 to advertise the aggregated 1.0.0.0/8 summary instead of the more-specific routes to Level 1 neighbors like R2.

Why the distractors are wrong:

• A (level-1-2) applies summarization to both Level 1 and Level 2, which is broader than what's needed and doesn't isolate the fix to Level 1 as the question requires.
• B (no summary-address) is a removal command - it would delete the summarization entirely, making the problem worse.
• D (level-2) summarizes routes injected into the Level 2 backbone, not within Level 1, so it wouldn't fix R2 receiving specific routes via Level 1.

Memory tip: Match the keyword to the IS-IS level where you want the summary to appear - think of it as "summarize when advertising into Level ___." If the traffic path is L1 → L2, you summarize at level-2; if you're keeping it within an L1 area, use level-1.

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