Cisco Transceivers and Cabling: SFP, QSFP, Single-Mode vs Multimode

Transceiver form factor determines speed: SFP carries up to 1 Gb/s, SFP+ 10 Gb/s, SFP28 25 Gb/s, QSFP+ 40 Gb/s, and QSFP28 100 Gb/s. Media choice is driven by distance. Copper twisted pair reaches 100 m, direct attach copper reaches a rack or two, multimode fiber reaches hundreds of metres and less as the bit rate rises, and single-mode fiber reaches tens of kilometres. Once a link leaves property you own, the question stops being which optic and becomes which WAN service.

Form factors

A transceiver is a hot-swappable module that slides into a switch or router cage and converts between the device’s electrical signaling and whatever the cable carries. One switch port can therefore be copper, short-reach multimode, or 40 km single-mode depending on which module you insert.

• SFP (Small Form-Factor Pluggable): 100 Mb/s and 1 Gb/s. Common variants are 1000BASE-T over copper, 1000BASE-SX over multimode, and 1000BASE-LX over single-mode.
• SFP+: 10 Gb/s in the same physical cage. Most SFP+ ports also accept an SFP module and run it at the lower speed.
• SFP28: 25 Gb/s, again in the same cage size. The 25 Gb/s lane is the building block of 100 Gb/s.
• QSFP+ (Quad SFP+): 40 Gb/s, built as four 10 Gb/s lanes. Physically larger than SFP and not interchangeable with it.
• QSFP28: 100 Gb/s, built as four 25 Gb/s lanes.

Because QSFP modules are internally four lanes, a breakout cable can split one 40 Gb/s port into four 10 Gb/s connections, or one 100 Gb/s port into four 25 Gb/s connections. That is how a single high-speed uplink port is subdivided to feed several servers or switches.

Direct attach and active optical cables

Three products sit between a bare transceiver and a finished cable assembly.

A DAC (direct attach copper) cable is twinaxial copper with the transceiver bodies permanently molded onto both ends. Passive DACs run to roughly 5 m and active DACs to roughly 10 m. They cost far less than a pair of optics, draw almost no power, and add negligible latency, which makes them the standard choice for server-to-top-of-rack links inside one rack or between adjacent racks, the densest cabling layer in a spine-leaf data center.

An AOC (active optical cable) is the same idea using fiber: optics permanently attached to a fixed length of fiber, typically up to 30 m or so. It reaches farther than DAC and weighs less, but the length is fixed at purchase and the ends cannot be cleaned or reterminated.

Separate transceivers with a patch cord is the field-serviceable option, and the one to use for anything leaving the rack. The optic and the cable are independent, the length is whatever the run needs, and a dirty connector can be cleaned rather than replaced.

Single-mode and multimode fiber

Multimode fiber has a large core, either 62.5 or 50 micrometres. A large core admits light at many different entry angles, and each angle is a propagation mode that follows a different physical path down the fiber. Longer paths take longer to arrive, so a pulse launched as a sharp square edge arrives smeared across time. That smearing is modal dispersion, and it is the reason multimode distance is limited.

Two consequences follow. The higher the bit rate, the shorter each bit period, so less smearing can be tolerated before adjacent bits overlap and the receiver cannot distinguish them: the same fiber that carries 1 Gb/s for 550 m may only carry 10 Gb/s for 82 m. And better multimode fiber is defined by a tighter modal bandwidth specification rather than by a different core size.

Single-mode fiber has a core around 9 micrometres, small enough that only one mode propagates. With modal dispersion eliminated, the limits become attenuation and chromatic dispersion, both of which are far gentler, so single-mode reaches from 10 km to 80 km and beyond. It uses narrow-linewidth lasers at 1310 nm or 1550 nm, while multimode uses inexpensive 850 nm VCSEL sources.

	Multimode	Single-mode
Core diameter	50 or 62.5 micrometres	About 9 micrometres
Limiting factor	Modal dispersion	Attenuation and chromatic dispersion
Light source	850 nm VCSEL, low cost	1310 nm or 1550 nm laser, higher cost
Typical reach	33 m to 550 m depending on grade and speed	10 km, 40 km, or 80 km by optic type
Cable cost	Lower	Slightly higher, but the optics dominate
Where used	Inside a building, data center rows, riser	Campus between buildings, metro, WAN
Common optics	1000BASE-SX, 10GBASE-SR, 40GBASE-SR4	1000BASE-LX, 10GBASE-LR, 10GBASE-ER

OM grades and distance

Multimode is graded OM1 through OM5. The grade sets the distance at each speed.

• OM1, 62.5/125, orange jacket. 1000BASE-SX to 275 m, 10GBASE-SR to only 33 m. Legacy installations.
• OM2, 50/125, orange jacket. 1000BASE-SX to 550 m, 10GBASE-SR to 82 m.
• OM3, 50/125 laser-optimized, aqua jacket. 10GBASE-SR to 300 m, 40GBASE-SR4 to 100 m, 100GBASE-SR4 to 70 m.
• OM4, aqua or violet jacket. 10GBASE-SR to 400 m, 40GBASE-SR4 to 150 m, 100GBASE-SR4 to 100 m.
• OM5, lime green, adds wideband support for short-wavelength multiplexing.

The pattern to remember rather than the table: on the same fiber, a tenfold increase in bit rate costs most of your distance. A 300 m OM3 run that works at 10 Gb/s does not work at 40 Gb/s.

Connectors

• LC (Lucent Connector): small form factor, latching, almost always duplex. This is what plugs into SFP, SFP+, and SFP28 modules, and into QSFP28 optics that multiplex wavelengths onto a single fiber pair such as LR4 and CWDM4.
• SC: larger square body with a push-pull latch. Found on patch panels, older equipment, and many provider handoffs.
• ST: round bayonet twist-lock. Legacy multimode installations.
• MPO/MTP: a single ferrule holding 12 or 24 fibers. Required by parallel optics, meaning 40GBASE-SR4 and 100GBASE-SR4, where four lanes transmit and four receive simultaneously over separate strands.

Duplex fiber uses one strand to transmit and one to receive, so a fresh run that shows no link is most often crossed. Swap the two strands at one end before troubleshooting anything else. A run that does link but performs badly is a different fault, and dirty connectors, an over-length span, or a marginal optical budget all announce themselves as rising CRC and input error counts in the interface counter fields.

Copper category limits

Balanced twisted pair is standardized to a 100 m channel that includes patch cords at both ends, and the category rating determines what speed survives that distance.

• Cat 5e: 1000BASE-T to 100 m. Not rated for 10GBASE-T.
• Cat 6: 1000BASE-T to 100 m, but 10GBASE-T to only 55 m, and less in tightly bundled runs. The limit exists because of alien crosstalk, which is interference coupling between adjacent cables rather than between pairs inside one cable, and Cat 6 does not specify it tightly enough for 10 Gb/s at full length.
• Cat 6a: 10GBASE-T to the full 100 m. The construction is thicker with improved separation and often shielding, specifically to control alien crosstalk.
• Cat 8: 25GBASE-T and 40GBASE-T to 30 m, intended for short data center runs.

If a requirement states 10 Gb/s over copper at a distance beyond 55 m, Cat 6a is the answer and Cat 6 is the trap.

Copper also carries a failure mode fiber does not. Autonegotiation runs over the same pairs, and when one end is pinned by hand while the other negotiates, the link comes up at matching speed with mismatched duplex and collapses under load. The symptoms of a duplex mismatch look like a bad cable and are not one.

Choosing media from a requirement

Read the distance first, then the bit rate.

Under 10 m in a rack at 10 Gb/s or above: DAC. Under 100 m at 1 or 10 Gb/s to a desk or a nearby closet: copper, choosing Cat 6a if 10 Gb/s is required. Between 100 m and roughly 400 m inside a building at 10 Gb/s: multimode, with the OM grade selected from the distance. Between buildings, or anything past 550 m, or anything at 40 km: single-mode.

WAN services beyond your own fiber

Once the two endpoints sit on separate properties, you cannot simply choose an optic, because you do not own the path. You also need a device with the right interface to terminate the circuit, which is one of the jobs that separates a router from a Layer 3 switch. Four service types cover the CCNA scope.

A point-to-point leased line delivers a fixed, dedicated amount of bandwidth on a circuit no other subscriber uses. The provider carries the customer’s frames without participating in the customer’s routing, so the two customer routers sit one Layer 3 hop apart and peer directly. A requirement specifying dedicated non-shared bandwidth plus no provider involvement in routing is satisfied only by this option.

An MPLS Layer 3 VPN gives any-to-any connectivity across a provider cloud, but the provider edge routers exchange routes with the customer edge routers by design. That is the service working correctly, and it is disqualifying whenever the provider must stay out of the routing.

Broadband Internet with a site-to-site IPsec VPN installs fastest and costs least, but the access circuit and the Internet path between sites are both shared, so no bandwidth is guaranteed. Encryption is mandatory because the transport is public.

A dial-on-demand or cellular circuit comes up only when traffic is queued for the far end and drops when idle. It suits backup and low-volume telemetry, not a continuous high-rate link.

How the 200-301 exam tests this

• Distance and speed to media. A stem gives a run length and a required rate and asks for the cabling. Watch for the 55 m Cat 6 ceiling at 10 Gb/s, for multimode distances collapsing as speed rises, and for anything past a few hundred metres pointing at single-mode.
• Form factor to speed. You are asked which module a 40 Gb/s or 100 Gb/s port takes. QSFP+ is 40, QSFP28 is 100, SFP+ is 10, SFP28 is 25, and an SFP cage cannot physically hold a QSFP.
• WAN service selection. The stem lists constraints such as fixed bandwidth, not shared, and no provider participation in routing. Test each option against every clause: MPLS L3VPN fails the routing clause, broadband fails the dedicated clause, dial-on-demand fails the continuous clause.
• Cost and serviceability. Short in-rack links at high speed favor DAC over a pair of optics, and the distractor is a single-mode pair for a 3 m connection.

The distances and form factors are raw memorization, and the only reliable check is retrieval — quiz the numbers with CCNA practice questions instead of rereading the table.

Quick reference

• SFP 1 Gb/s, SFP+ 10 Gb/s, SFP28 25 Gb/s, QSFP+ 40 Gb/s, QSFP28 100 Gb/s.
• QSFP modules are four lanes internally and can be broken out into four lower-speed links.
• DAC to roughly 5 m passive or 10 m active; AOC to roughly 30 m; separate optics plus patch cord for everything else.
• Multimode is limited by modal dispersion, so distance falls sharply as bit rate rises.
• 10GBASE-SR: OM1 33 m, OM2 82 m, OM3 300 m, OM4 400 m. 40GBASE-SR4: OM3 100 m, OM4 150 m.
• Single-mode has a 9 micrometre core, no modal dispersion, and reaches 10 km, 40 km, or 80 km by optic.
• LC on SFP-family modules, SC on panels and older gear, MPO on parallel 40G and 100G optics.
• Cat 6 carries 10GBASE-T only to 55 m because of alien crosstalk; Cat 6a carries it the full 100 m.
• A leased line is dedicated and keeps the provider out of your routing; MPLS L3VPN has the provider peering with you by design.