You have a customer who wants 3:1 oversubscription and has a CX switch with 24 ports, and each port operates at 25 Gbps edge ports. What is the expected uplink speed?

Question

Options

• A400 Gbps
• B250 Gbps
• C100 Gbps
• D200 Gbps

• A
  3% (1)
• B
  11% (4)
• C
  6% (2)
• D
  81% (29)

Explanation

D (200 Gbps) is correct because the total edge bandwidth is 24 ports × 25 Gbps = 600 Gbps, and with a 3:1 oversubscription ratio, you divide that by 3: 600 ÷ 3 = 200 Gbps uplink.

Why the distractors are wrong:

• A (400 Gbps) would only represent a 1.5:1 oversubscription (600 ÷ 400), not 3:1
• B (250 Gbps) implies a 2.4:1 ratio - a real-ish number but doesn't match the given 3:1
• C (100 Gbps) represents a 6:1 oversubscription (600 ÷ 100), doubling the required ratio

Memory tip: Think of the oversubscription ratio as a divisor for the uplink: Uplink = (Ports × Port Speed) ÷ Oversubscription. Higher oversubscription = smaller (cheaper) uplink, which means more contention - the "over" in oversubscription means you're selling more bandwidth than you actually have.

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