Advertising a Default Route from Core to Aggregation to Hide Instability

Design multicampus Layer 3 infrastructures

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

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? -

The aggregation layer must advertise a default route toward the core layer.
The core layer must advertise a default route toward the aggregation layer. Correct Answer
The aggregation layer must advertise a default route toward the access layer.
The core layer must advertise a default route toward the aggregation layer.

Community Votes

62%

38%

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

Community Insight

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.

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.

Community Discussion (5 comments)

muffedtrims 👍 1 Selected: B

I agree with 08511d4, correct answer is B

08511d4 👍 4 Selected: 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."

salmarin 👍 2 Selected: C

think C

RexChen 👍 1 Selected: C

I think it's C

TheGorn 👍 1

A /17 summary could keep the two environments separated.

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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.

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