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352-001 · Question #327

Which two options are two advantages of summarizing networks at the aggregation layer rather than at the core? (Choose two.)

The correct answer is A. It prevents the core from having unnecessary routes. D. It avoids network-wide impact upon VLAN changes local to the aggregation devices.. Summarizing at the aggregation layer keeps core routing tables lean and prevents local VLAN changes from propagating to the wider network.

Designing Network Infrastructure

Question

Which two options are two advantages of summarizing networks at the aggregation layer rather than at the core? (Choose two.)

Options

  • AIt prevents the core from having unnecessary routes.
  • BIt no longer needs a core layer.
  • CIt prevents black hole routing.
  • DIt avoids network-wide impact upon VLAN changes local to the aggregation devices.

How the community answered

(19 responses)
  • A
    74% (14)
  • B
    16% (3)
  • C
    11% (2)

Why each option

Summarizing at the aggregation layer keeps core routing tables lean and prevents local VLAN changes from propagating to the wider network.

AIt prevents the core from having unnecessary routes.Correct

When summarization is applied at the aggregation layer, only aggregate prefixes are advertised upward to the core, preventing it from learning every individual subnet route. This reduces core routing table size and lowers convergence overhead on core routers.

BIt no longer needs a core layer.

Summarization reduces route information passed upward but does not remove the functional need for a core layer to provide high-speed interconnection between aggregation blocks.

CIt prevents black hole routing.

Improper summarization can actually cause black hole routing when a summary route exists but no more-specific matching route is present for a given destination, causing silent packet drops.

DIt avoids network-wide impact upon VLAN changes local to the aggregation devices.Correct

Summary routes hide the details of specific subnets and VLANs within an aggregation block, so adding or changing VLANs locally does not trigger routing updates throughout the rest of the network. This confines the impact of topology changes to the aggregation layer where they originate.

Concept tested: Route summarization benefits at aggregation layer

Source: https://www.cisco.com/c/en/us/support/docs/ip/ip-routing/13730-summarization.html

Topics

#route summarization#aggregation layer#black hole routing#network hierarchy

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