Domain 1 of 5 · Chapter 8 of 9

Network Topologies

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Included in this chapter:

  • The redundancy-versus-cost tradeoff and physical vs logical
  • Mesh and star: the redundancy extremes
  • Hierarchical designs: three-tier and collapsed core
  • Spine-and-leaf for the data center, plus hybrid

Topology families compared on redundancy, cost, scalability, and fault domain

CriterionFull meshStar / hub-and-spokeThree-tierSpine-and-leaf
RedundancyHighest (every node interconnected)Low (one hub)High at core/distribution via redundant linksHigh (multiple equal paths leaf-to-leaf)
Link count / costn(n-1)/2 links, most expensiveOne link per node, cheapestModerate, layered uplinksEvery leaf to every spine, moderate-high
Single point of failureNoneCentral hub/switchMitigated by redundant core/distributionNone for east-west paths
Scales byAdding links (grows fast)Adding ports to the hubAdding distribution/access blocksAdding spines (bandwidth) and leaves (ports)
Best traffic patternAny-to-any critical linksHub-centricNorth-south (client-server)East-west (server-server)

Decision tree

Only two endpoints toconnect?YesPoint-to-pointNoData center, mostlyeast-west traffic?YesSpine-and-leaf2 hops, uniform latencyNoMax redundancy, nosingle point of failure?YesFull meshpartial mesh if cost matters tooNoLarge campus needinglayer separation?YesThree-tiercollapsed core if smallNoStar / hub-and-spokecheapest; hub is the SPOFReal networks are hybrid: mix shapes where each fits (hub-and-spoke WAN over three-tier campuses with a spine-leaf data center).Physical topology = cabling; logical topology = how data actually flows (a switched star is logically a bus).
Cheat sheet

Cheat sheet

  • A full mesh needs n(n-1)/2 links and has no single point of failure
  • Use a partial mesh to add redundancy only on the paths that matter
  • A star's central hub or switch is its single point of failure
  • Hub-and-spoke is a star at WAN scale, with the central site as the SPOF
  • Point-to-point connects exactly two endpoints over one dedicated link
  • Physical topology is the cabling; logical topology is how data flows
  • The three-tier model layers a network into core, distribution, and access
  • Routing, filtering, and QoS policy belong in the distribution layer
  • Collapsed core merges the core and distribution layers for smaller networks
  • In spine-and-leaf, every leaf connects to every spine and never to another leaf
  • Spine-and-leaf makes any server-to-server path two hops with uniform latency
  • Spine-and-leaf scales bandwidth by adding spines and ports by adding leaves
  • East-west traffic suits spine-leaf; north-south suits the hierarchy
  • Hybrid is the right label when a design deliberately mixes topologies
  • Redundancy decisions are fault-domain decisions
  • Spine-leaf uses ECMP so every uplink forwards at once with no blocked standby links
  • Three-tier hierarchy adds latency because traffic climbs access, distribution, and core layers

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References

  1. CompTIA Network+ (N10-009) Certification – Exam Objectives
  2. Cisco Hierarchical Campus Network Design (core / distribution / access)
  3. RFC 7938 – Use of BGP for Routing in Large-Scale Data Centers (folded Clos / fat-tree) Whitepaper