Switch roles and topologies: where each optic ends up

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The same 100G module is a cheap DAC in one rack, an SR4 across the room, a LR4 between buildings and a DWDM channel between cities. Which one a port gets is decided by the switch's role in the network and the topology around it. This page maps the classic campus tiers and the data-centre leaf–spine to the optics, distances and counts they imply — so a bill of materials can be read off the design.

Campus three-tier hierarchy and data-centre leaf–spine with the optics on each link

Campus: access, distribution, core

Tier	Role	Typical ports	Downlinks	Uplinks and optics
Access	connects end devices	24–48 × RJ45 (1G/mGig, PoE) + 2–4 SFP+/SFP28 uplinks	copper	10G/25G SR (in-building MMF), LR (between buildings), sometimes 1G LX
Distribution / aggregation	aggregates access switches per building or floor group	24–48 × SFP+/SFP28, 4–8 × QSFP28	10G/25G SR/LR	40G/100G SR4/LR4 to core; CWDM/DWDM if fibre is scarce
Core	interconnects distribution, routes	QSFP28/QSFP-DD	100G/400G	LR4/FR4/ER4 between sites; CWDM or DWDM over leased fibre
Collapsed core	small sites: distribution + core in one	mixed	—	—

Campus distances are metres to a few kilometres: SR on OM3/OM4 inside buildings, LR/LX on single-mode between them; the reach classes in PMD reference.

Data centre: leaf–spine (Clos)

Role	Also called	Ports	Downlinks	Uplinks
Leaf	ToR (top of rack), access	48 × SFP28/SFP56 + 8 × QSFP28/QSFP-DD, or 32 × QSFP-DD	servers: 25G/50G/100G — DAC in rack (≤ 3 m), AOC/SR to adjacent racks	to every spine: 100G/400G SR4/DR4/AOC (≤ 100 m)
Spine	aggregation, fabric	32–64 × QSFP28/QSFP-DD/OSFP	leaves	to super-spine or border: DR4/FR4 (500 m–2 km)
Super-spine / core	pod interconnect	high-radix 400G/800G	spines	DR/FR within the campus; ZR/ZR+ for DCI (Coherent)
Border leaf	edge to WAN/internet	mixed	—	LR/ER/CWDM/DWDM to carriers
Management	out-of-band	1G RJ45/SFP	BMCs, consoles	1G/10G LR to the OOB core

Rules of thumb: leaf–spine links are all the same speed and FEC; oversubscription (downlink capacity ÷ uplink capacity) of 3:1 is common for general compute, 1:1 for AI and storage (GPU fabrics).

Rail-optimised AI fabrics

GPU nodes connect each NIC to a different leaf, so most server links leave the rack — AOC and optics instead of DAC — and the cable count per GPU is one per tier (Cabling an AI cluster).

Storage and SAN

Fibre Channel switches form their own fabric (edge–core or core–edge–core) with 16/32/64GFC SW optics in a room and LW between rooms; Ethernet storage rides the leaf–spine with lossless classes (SAN design, Lossless Ethernet).

Distance → optic

Segment	Distance	Medium	Optic
Server to ToR	≤ 3 m	copper	DAC (Cable internals)
Server to end-of-row leaf	3–30 m	AOC or MMF	AOC, SR
Leaf to spine, same hall	10–100 m	OM4 or SMF	SR4/SR8, DR4/DR8
Between halls / buildings	100 m – 2 km	SMF	DR, FR4, LR/LX
Campus / metro	2–40 km	SMF	LR, ER, CWDM, 4WDM
Regional DCI	40–120 km	SMF, amplified	ZR/ZR+, DWDM transponders
Long haul	> 120 km	SMF, amplified	DWDM line systems (Network design & OTN)

Fibre-scarce paths use CWDM or DWDM to multiply capacity per strand; single-fibre BiDi where only one strand exists.

Counting optics from a design

Example: one pod of 16 racks, 32 servers per rack, 2 × 25G per server, 4 spines.

Item	Count
Server links (2 × 25G × 32 × 16)	1 024 DAC
Leaf uplinks (16 leaves × 4 spines × 2 × 100G)	128 links → 256 QSFP28 SR4/DR4 + 128 MPO trunks
Spine to super-spine (4 spines × 8 × 100G)	32 links → 64 modules
Border + OOB	8–16 LR modules, 48 1G
Spares (5 %)	~20

Multiply by two for the second fabric if the design is dual-plane.

Roles and policy

Role also decides how strict the platform is about optics: carrier and SAN gear tends to enforce OEM branding; data-centre leaf–spine on merchant silicon is often permissive; campus access varies by vendor (How each NOS validates a module, Vendor lock).

In CodingBox

A role-based bill of materials is a list of PMDs and part numbers; CodingBox verifies each delivered module against it (type, reach, wavelength, PN) and codes identities where the design calls for OEM-compatible modules (Check transceiver, Code database).

Turning a design into a purchase and a working rack — selection criteria, optics bill of materials, commissioning checklist and acceptance test: Selecting & commissioning; the L2/L3 mechanics behind these topologies: Forwarding basics.

The cabling under these topologies — structured cabling hierarchy, data-centre patterns (EoR, MoR, ToR, trunks and cassettes), fibre-count planning: Fibre network topologies.

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