400-007 · Question #161
400-007 Question #161: Real Exam Question with Answer & Explanation
The correct answer is D: at the aggregation layer because it is the Layer 2 to Layer 3 demarcation point. In a three-layer design with a Layer 3-only core, the aggregation layer is the optimal termination point for a Layer 2 interconnection because it serves as the boundary between Layer 2 and Layer 3 domains.
Question
Options
- Aat the access layer because the STP root bridge does not need to align with the HSRP active
- Bat the core layer, to offer the possibility to isolate STP domains
- Cat the core layer because all external connections must terminate there for security reasons
- Dat the aggregation layer because it is the Layer 2 to Layer 3 demarcation point
Explanation
In a three-layer design with a Layer 3-only core, the aggregation layer is the optimal termination point for a Layer 2 interconnection because it serves as the boundary between Layer 2 and Layer 3 domains.
Common mistakes.
- A. Terminating at the access layer would extend STP domains unnecessarily deep into the network, increasing convergence complexity and failure blast radius rather than reducing it.
- B. The core in this design is explicitly Layer 3 only, making it technically incapable of terminating a Layer 2 interconnection regardless of STP domain isolation benefits.
- C. There is no valid design principle requiring all external connections to terminate at the core for security - this is not supported by Cisco data center hierarchical design guidelines.
Concept tested. Layer 2 interconnection termination point in three-tier DC design
Reference. https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Data_Center/DC_Infra2_5/DCinf_ch3.html
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