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352-001 · Question #770

Company A, a global content provider owns, data centers on different continents. Their data center design involves a standard three-Layer design with a Layer 3-only. HSRP is used as the FHRP. They…

The correct answer is A. at the aggregation layer because it is the Layer 2 to Layer 3 demarcation point. In a three-layer data center design with a Layer 3-only core, the aggregation layer serves as the Layer 2/Layer 3 boundary and is the optimal termination point for a Layer 2 interconnection.

Designing Network Infrastructure

Question

Company A, a global content provider owns, data centers on different continents. Their data center design involves a standard three-Layer design with a Layer 3-only. HSRP is used as the FHRP. They require VLAN extension across access switches in all data centers, and they plan to purchase a Layer 2 interconnection between two of their data centers in Europe. In the absence of other business or technical constraints, which termination point is optimal for the Layer 2 interconnection?

Options

  • Aat the aggregation layer because it is the Layer 2 to Layer 3 demarcation point
  • Bat the core layer because all external connections must terminate there for security reasons
  • Cat the core layer, to offer the possibility to isolate STP domains
  • Dat the access layer because the STP root bridge does not need to with the HSRP

How the community answered

(23 responses)
  • A
    57% (13)
  • B
    9% (2)
  • C
    13% (3)
  • D
    22% (5)

Why each option

In a three-layer data center design with a Layer 3-only core, the aggregation layer serves as the Layer 2/Layer 3 boundary and is the optimal termination point for a Layer 2 interconnection.

Aat the aggregation layer because it is the Layer 2 to Layer 3 demarcation pointCorrect

The aggregation layer is the Layer 2 to Layer 3 demarcation point in a standard hierarchical three-layer design. HSRP runs at this layer because it bridges L2 VLANs to L3 routing, making it the natural anchor for any L2 extension. Terminating the Layer 2 interconnect here keeps STP domains contained within the access-aggregation boundary and avoids introducing L2 into the L3-only core.

Bat the core layer because all external connections must terminate there for security reasons

The core is Layer 3-only in this design and does not participate in Layer 2 switching, so it cannot serve as a Layer 2 termination point regardless of any security policy.

Cat the core layer, to offer the possibility to isolate STP domains

Terminating at the L3-only core is architecturally incorrect; STP domain isolation is achieved by keeping L2 at the aggregation layer, not by extending it upward to the core.

Dat the access layer because the STP root bridge does not need to with the HSRP

STP root bridge placement is a separate design concern and does not dictate where a Layer 2 interconnect should terminate; the aggregation layer remains the correct L2/L3 demarcation point.

Concept tested: Layer 2 WAN termination in three-tier data center design

Source: https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Data_Center/DC_3_0/DC-3_0-CCDesign.html

Topics

#data center interconnect#Layer 2 extension#aggregation layer#STP

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