HPE7-A04 · Question #10
You are involved in a 2-Tier L2 Data Center Design with the following details: - CX 8325 100G Core Switches - CX 10000 48-P 25G Access Switches - 100G SR4 Optical connections between Core and Access…
The correct answer is A. Solution can scale to 7 total racks using the same subscription rates. Option A is correct because the math works out to exactly 7 total racks. Each core switch has 13 available 100G ports, but 3 are consumed by VSX overhead (2 for ISL + 1 for Keepalive), leaving 10 ports for access uplinks. Since each access rack requires 2 x 100G SR4 uplinks…
Question
You are involved in a 2-Tier L2 Data Center Design with the following details:
- CX 8325 100G Core Switches
- CX 10000 48-P 25G Access Switches
- 100G SR4 Optical connections between Core and Access
In this design, both the Core Layer and Access Layer consist of pairs of switches configured in VSX for HA. Core Switches = VSX Pairs of CX 8325 32-port 100G switches (B-F airflow) Each pair uses:
- 2 x 100G QSFP28 1M DACs for ISL
- 1 x 100G QSFP28 1M DAC for Keepalive
- 2 x 100G QSFP28 SR4 Optic to each access switch
Each Core switch has 13 available 100G QSFP28 interfaces. What is the maximum number of racks supported by this design?
Options
- ASolution can scale to 7 total racks using the same subscription rates.
- BSolution can scale to 14 total racks using the same subscription rates.
- CSolution can scale to 8 total racks using varying subscription rates.
- DSolution can scale to 16 total racks using the same subscription rates.
How the community answered
(16 responses)- A56% (9)
- B25% (4)
- C13% (2)
- D6% (1)
Explanation
Option A is correct because the math works out to exactly 7 total racks. Each core switch has 13 available 100G ports, but 3 are consumed by VSX overhead (2 for ISL + 1 for Keepalive), leaving 10 ports for access uplinks. Since each access rack requires 2 x 100G SR4 uplinks from each core switch, that supports 10 ÷ 2 = 5 access racks. Adding the 2 core switch racks (one per CX 8325 in the VSX pair) yields 5 + 2 = 7 total racks, using consistent 1:1 uplink ratios throughout.
Option B (14 racks) is wrong because it likely ignores the VSX overhead ports (ISL + Keepalive) and incorrectly treats each core switch as serving independent racks rather than both connecting to the same access racks - a fundamental misunderstanding of VSX topology.
Option C (8 racks with varying rates) is wrong because it requires oversubscription, which is not present in this design; the 2 x 100G uplinks per access rack maintain a consistent subscription ratio.
Option D (16 racks) is wrong for a similar reason as B - it overcounts by treating the two core switches as serving separate, non-overlapping sets of access racks, when in reality both core switches uplink to the same set.
Memory tip: Use the formula "13 − 3 = 10 ÷ 2 = 5 access + 2 core = 7" - always subtract VSX overhead (ISL×2 + KA) before dividing by uplinks-per-rack, then add the core racks last.
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