Examkin

Question

Difficulty: Very hardNetwork Topology Architectures

An enterprise network architect is designing a two-tier Spine-Leaf (Clos) topology for a data center hosting high-performance containerized workloads. The design deploys 4 Spine switches and multiple Leaf switches. Each Leaf switch is configured with 48 x 25GbE downlink ports for server connections and 4 x 100GbE uplink ports, with exactly one uplink connecting to each of the 4 Spine switches. Layer 3 routing with Equal-Cost Multi-Pathing (ECMP) is configured across all spine-leaf interconnections. Which statement correctly evaluates the architectural traits and bandwidth performance of this Spine-Leaf deployment?

  1. The Leaf switch layer exhibits a 3:1 oversubscription ratio for East-West traffic while delivering deterministic, equal-latency forwarding across the spine fabric between any two Leaf switches.Answer
  2. B
    Spanning Tree Protocol (STP) must be enabled on all spine-to-leaf trunk interfaces to block redundant physical links and prevent Layer 2 loops across the leaf switches.
  3. C
    The Leaf switches act as Layer 2 bridges that transparently extend broadcast domains through the Spine switches, removing the need for routing at the leaf-spine interconnect.
  4. D
    Adding a bare-metal Type 1 hypervisor layer between Leaf and Spine switches is required to manage ECMP routing tables and calculate dynamic uplink oversubscription.

Answer

The Leaf switch layer exhibits a 3:1 oversubscription ratio for East-West traffic while delivering deterministic, equal-latency forwarding across the spine fabric between any two Leaf switches.
The total host downlink bandwidth per Leaf switch is 48×25 Gbps=1200 Gbps48 \times 25\text{ Gbps} = 1200\text{ Gbps}, while the total fabric uplink bandwidth is 4×100 Gbps=400 Gbps4 \times 100\text{ Gbps} = 400\text{ Gbps}. Dividing downlink by uplink capacity yields an oversubscription ratio of 1200:4001200:400, which simplifies to 3:13:1. Additionally, because every Leaf switch connects directly to all 4 Spine switches in a two-tier Clos architecture, any inter-leaf traffic takes exactly 2 switch hops (Leaf →\rightarrow Spine →\rightarrow Leaf), guaranteeing consistent, deterministic latency across equal-cost paths.

Step-by-Step Solution

1
Calculate total host-facing downlink capacity per Leaf switch.
48 ports × 25 Gbps = 1200 Gbps total downlink bandwidth.
Host access capacity is determined by multiplying the number of downlink access ports by their individual link speed.
2
Calculate total fabric-facing uplink capacity per Leaf switch.
4 ports × 100 Gbps = 400 Gbps total uplink bandwidth.
Fabric uplink capacity is determined by multiplying the number of uplink connections to the spine by their link speed.
3
Determine the oversubscription ratio of the Leaf layer.
1200 Gbps : 400 Gbps = 3:1 oversubscription ratio.
Oversubscription ratio is calculated as Total Ingress (Downlink) Bandwidth divided by Total Egress (Uplink) Bandwidth.
4
Analyze path latency and forwarding behavior of the Spine-Leaf (Clos) fabric.
Every leaf-to-leaf path is exactly 2 fabric hops (Leaf A → Spine → Leaf B) using Layer 3 ECMP across all available Spine switches.
Spine-Leaf architectures guarantee consistent East-West latency because all endpoints connected to different leaf switches are separated by the exact same number of switch hops.

Key Concept

Spine-Leaf Architecture Oversubscription and ECMP Operation
Rate this question

Topics

Network Topology Architectures

More questions from this topic