What is a rule of thumb for oversubscription ratio on the ToR switches for a two-tier network design?

Question

Options

• A12:1-14:1
• B1:1-2:1
• C6:1-10:1
• D3:1-5:1

• A
  1% (1)
• B
  6% (4)
• C
  1% (1)
• D
  91% (63)

Explanation

D (3:1–5:1) is the industry-standard rule of thumb for ToR switch oversubscription in a two-tier network because it balances cost efficiency with acceptable performance - servers share uplink bandwidth at a reasonable ratio that avoids bottlenecks without requiring expensive 1:1 provisioning.

Why the distractors are wrong:

• A (12:1–14:1) is far too aggressive for ToR switches and would cause severe congestion; ratios this high are sometimes seen in older three-tier designs at the aggregation layer, not at the access/ToR layer.
• B (1:1–2:1) represents near line-rate (non-blocking or minimally oversubscribed) design, which is ideal but prohibitively expensive for most deployments; it's associated with high-performance or latency-sensitive environments, not general two-tier rules of thumb.
• C (6:1–10:1) is too high for a ToR switch and could indicate a misremembering of aggregation-layer ratios or older campus network guidelines.

Memory tip: Use the phrase "Three-to-Five for ToR" - the alliteration links the 3:1–5:1 range directly to Top-of-Rack switches, making it easy to recall under exam pressure.

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