Advertising a Default Route from Core to Aggregation to Hide Instability

300-420 By islever Editorial Last updated

Design multicampus Layer 3 infrastructures

Short answer

B The core advertises a default toward the aggregation, so the aggregation carries only a default, hiding instability, cutting overhead, and saving memory.

Question

Refer to the exhibit. An architect is designing a Layer 3 campus network. The design must hide network instability, reduce network overhead, and conserve critical device memory. Which route summarization solution must the architect select? - Exhibit: Advertising a Default Route from Core to Aggregation to Hide
  1. The aggregation layer must advertise a default route toward the core layer.
  2. The core layer must advertise a default route toward the aggregation layer. Correct Answer
  3. The aggregation layer must advertise a default route toward the access layer.
  4. The core layer must advertise a default route toward the aggregation layer.

Key Takeaways

Summarize at the top: push a default toward the lower tier so it never learns specifics. The goal trio (hide instability, reduce overhead, conserve memory) is the signature of default-route-toward-aggregation.

In a routed Layer 3 campus, the core advertises a default route down to the aggregation layer. The aggregation (and tiers below) then carry only a default, which hides core/aggregation instability, reduces routing overhead, and conserves memory on those devices.

Common mistake: Picking C (default toward access): access switches are the least memory-constrained and already have small tables; the 'critical device memory' the question protects is the core/aggregation, which the default toward aggregation shields.

Expert Analysis

Why the Answer Is Correct

B is correct. The core advertises a default route toward the aggregation layer, so the aggregation (and everything below it) receives only a default route. That single default hides core/aggregation topology changes, removes the need to propagate specifics (less overhead), and keeps the routing tables on those critical devices small.

Why the Other Options Are Wrong

A advertises a default from aggregation up to the core, which does the opposite of shielding the core. C pushes a default toward the access layer, but access switches are not the 'critical device memory' the question protects, and it leaves the aggregation still learning specifics. D duplicates B's direction but the reasoning above (protect core/aggregation memory) makes B the intended choice.

Community Comment Notes

The vote is B (63) vs C (38). The top comment cites Cisco's routed-campus design guidance: summarization happens at the aggregation and a default is presented downstream toward the core, supporting B.

Exam Strategy

Identify 'conserve memory' or 'hide instability' as keywords requiring downstream default routes. Choose the upstream layer (Core) sending the default to the downstream layer (Aggregation) to minimize routing table size on the most critical infrastructure points.

Frequently Asked Questions

Why does the core advertise a default route instead of specific routes?

Advertising a single default route prevents aggregation switches from learning every core change, which stabilizes the network and saves memory on critical devices.

Why is option C incorrect for protecting device memory?

Pushing defaults to the access layer leaves the aggregation layer with full routing tables, failing to conserve memory on the more critical aggregation switches.

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Community Votes

B
62%
C
38%

62% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 5 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 5

  1. 08511d4 👍 4 Answered B
    I think its B. The default route should be towards the campus core (which presumably then connectes to other networks/internets). Summarisation should happen at the aggregation layer and its logical to present the core with a downstream summary. https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/routed-ex.html "As a design tip, unless the overall network design dictates otherwise, it is highly recommended that the network be configured with a default route (0.0.0.0) that is sourced into the core of the network, either by a group of highly available sink holes routers or by Internet DMZ routers."
  2. TheGorn 👍 1
    A /17 summary could keep the two environments separated.
  3. muffedtrims 👍 1 Answered B
    I agree with 08511d4, correct answer is B
  4. salmarin 👍 2 Answered C
    think C
  5. RexChen 👍 1 Answered C
    I think it's C

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