Cisco 1G SFP Guide: GLC-SX-MMD vs GLC-LH-SMD vs GLC-TE Explained

GLC-SX-MMD is Cisco's 1000BASE-SX multimode SFP (850nm, up to 550m on OM3/OM4 fiber, dual LC), while GLC-LH-SMD is the 1000BASE-LX/LH module (1310nm) rated for 10km on single-mode fiber and 550m on multimode with a mode-conditioning patch cord. Both ship with DOM (digital optical monitoring) as standard. Pick SX for short in-building multimode runs and LH-SMD for single-mode links to 10km or mixed-fiber flexibility.

Cisco's Gigabit Ethernet SFP catalog looks simple until you're staring at a pick list of GLC-SX-MMD, GLC-LH-SMD, GLC-ZX-SMD, GLC-TE, and GLC-BX-D/U with a purchase order due today. Every one of these is a genuine, currently shipping Cisco PID, and every one solves a different fiber-plant or distance problem. Order the wrong one and you get a module that link-lights but never passes traffic, or one that works in the lab and fails the moment you patch it into the real single-mode run between buildings. This guide breaks down the full 1G SFP family, what separates single-mode from multimode, why the trailing "D" matters, and where each PID belongs in a Catalyst, Nexus, or ISR deployment.

What's the difference between GLC-SX-MMD and GLC-LH-SMD?

The core distinction is fiber type and reach. GLC-SX-MMD is a 1000BASE-SX transceiver built for multimode fiber (MMF) at an 850nm wavelength. Over legacy OM1/OM2 fiber it's good for roughly 220-275m; over modern OM3/OM4 laser-optimized multimode, Cisco rates it up to 550m. It's the standard choice for intra-building and intra-rack links where multimode is already pulled — think wiring closets, data center top-of-rack to end-of-row, and campus buildings under a few hundred meters apart.

GLC-LH-SMD is the 1000BASE-LX/LH transceiver operating at 1310nm. It's rated for up to 10km on single-mode fiber (SMF), and — because Cisco engineered it as a dual-fiber-type module — it also runs on multimode fiber up to 550m when paired with a mode-conditioning patch cord (MCPC) at both ends. That MCPC requirement trips people up constantly: plug GLC-LH-SMD straight into multimode without the patch cord and you'll see excessive attenuation and unstable links from modal dispersion, even though the port comes up. If your run is genuinely single-mode, skip the MCPC question entirely and just terminate normally — the patch cord is a multimode-only accommodation.

Cisco single mode SFP options: LH-SMD, ZX-SMD, and BX bidi

"Cisco single mode SFP" isn't one part number — it's a family that trades distance for cost and fiber count. Here's how the three single-mode options stack up:

• GLC-LH-SMD — 1310nm, up to 10km on single-mode, the default choice for building-to-building and MDF-to-IDF runs on campus.
• GLC-ZX-SMD — 1550nm, rated up to 70km (and up to 100km on a good link budget with quality fiber), the long-haul option for metro-scale or dark-fiber WAN links between distant sites.
• GLC-BX-D / GLC-BX-U — 1000BASE-BX10 bidirectional ("bidi") transceivers that run duplex 1G traffic over a single strand of single-mode fiber instead of two. GLC-BX-D transmits at 1490nm and receives at 1310nm; GLC-BX-U is its mirror image, transmitting at 1310nm and receiving at 1490nm. They must be deployed as a matched D/U pair, one on each end, up to 10km. Bidi pairs exist specifically to double the usable capacity of a fiber plant with only one strand available per link — common in old conduit runs or repurposed CATV fiber.

The practical decision tree is short: multimode fiber already in the wall, distance under 550m — GLC-SX-MMD. Single-mode fiber, distance under 10km — GLC-LH-SMD. Single-mode, distance beyond 10km out to 70-100km — GLC-ZX-SMD. Single strand of single-mode fiber available (not a pair) — GLC-BX-D paired with GLC-BX-U. Copper Ethernet instead of fiber — GLC-TE.

What is GLC-TE and when do you use a copper SFP instead of fiber?

GLC-TE is Cisco's 1000BASE-T copper SFP: an RJ-45 module that plugs into an SFP cage and negotiates 10/100/1000 over Category 5e/6 cable up to 100m, with Auto-MDI/MDIX so you don't need a crossover cable. It's the go-to when you have an SFP-only uplink port (common on switch stacking modules, some WAN edge routers, and older Catalyst line cards) but need to hand off to copper — a firewall, an access point, or a legacy device with an RJ-45 NIC. GLC-TE is the extended-temperature-range successor to the older GLC-T, which Cisco has been phasing toward end of sale; for new deployments, spec GLC-TE. One caveat that catches people every time: GLC-TE does not support 10/100 fallback on every platform and generally performs best in switch-to-switch or switch-to-router 1000BASE-T links — verify the port's supported speeds in the platform's release notes if you need sub-gig fallback.

Are Cisco 1G SFPs compatible across Catalyst, Nexus, and ISR platforms?

Mostly yes, with platform-specific caveats. GLC-SX-MMD, GLC-LH-SMD, GLC-ZX-SMD, GLC-BX-D/U, and GLC-TE are qualified across the current Catalyst 9000 series (9200/9300/9400/9500), Nexus 9000/3000 switches with 1G-capable SFP ports, and ISR 4000-series and ASR routers with SFP WAN or LAN uplinks. The one place to double-check before ordering is the specific hardware SKU and IOS/NX-OS release — Cisco enforces optics qualification through the Cisco Optics-to-Device Compatibility Matrix (tmgmatrix.cisco.com), and a handful of dense-port line cards or older fixed-configuration switches only support a subset of the 1G family, particularly around bidi (GLC-BX) support and mixed-speed SFP+ cages that autonegotiate down to 1G. Always confirm against the matrix for your exact platform and code version rather than assuming parity across the family — it takes two minutes and prevents a DOA support case that's actually a compatibility gap.

What are the most common mistakes buying Cisco 1G SFPs?

• Ordering SX for a single-mode run. GLC-SX-MMD physically will not link over single-mode fiber cores at any usable distance — the wrong fiber-fiber connector matchup is the single most common return reason.
• Skipping the mode-conditioning patch cord. GLC-LH-SMD over multimode without an MCPC produces intermittent, distance-sensitive link flaps that look like a bad cable or bad transceiver but are actually modal dispersion — a known, documented limitation, not a defect.
• Mismatching a bidi pair. GLC-BX-D on one end requires GLC-BX-U on the other. Two GLC-BX-D modules (or two BX-U) will not link — the wavelengths talk past each other.
• Assuming non-genuine or gray-market optics pass Cisco's software checks. Many current Catalyst and Nexus releases run active optics validation and will log warnings or, on some platforms, refuse to bring up unrecognized transceivers. Genuine Cisco PIDs avoid that entirely.
• Buying GLC-T instead of GLC-TE for new builds. GLC-T is the legacy, lower-temperature-rated part heading toward end of sale; GLC-TE is the current recommended copper SFP.

For a deeper dive into cross-platform optics qualification, third-party compatibility risk, and how the Cisco Optics-to-Device Compatibility Matrix actually works, see our transceiver compatibility guide. To browse and configure genuine Cisco 1G, 10G, and higher-speed optics for your platform, head to optics and transceivers in the catalog.

Buying genuine Cisco optics through Uniqcli

As an authorized Cisco partner, we sell factory-genuine GLC-series optics — not gray-market or third-party compatible parts — which matters for warranty coverage, TAC support eligibility, and passing Cisco's optics validation checks on current IOS-XE and NX-OS releases. We typically quote below list price, and for government and education buyers we handle Smart Account association and TAA-compliance paperwork as part of the order so procurement doesn't stall on documentation.