Will This Optic Work in That Switch? The Three Checks, in Order
Written and maintained by Haink's network infrastructure team · Verified against Cisco documentation, 22 August 2026 · authorized-channel, serial-verified
Transceiver compatibility is three separate questions that people ask as one. Does the module physically fit and is it supported on this platform? Does the software running on the switch know about it? And will it actually link with the module at the far end? Each has a different authority, and getting a "yes" to the first two does not guarantee the third.
Check one: the platform
Cisco maintains a live tool rather than documents. The Optics-to-Device Compatibility Matrix — historically the TMG Matrix, at tmgmatrix.cisco.com and now also optics.cisco.com — answers whether a given transceiver is supported in a given switch, router or line card, and from which software version.
It is worth knowing that this replaced something. Cisco's own Catalyst 9300 release notes still point at an older URL of per-platform compatibility tables, and those tables no longer exist there — the page now carries only the user manuals for the two matrix tools. So if a supplier cites a static per-platform compatibility PDF, it is likely years out of date.
Check two: the software on the switch
This is the check that catches genuine Cisco optics being rejected, and it runs on the box itself:
show interfaces transceiver supported-list
The output lists transceiver types together with the minimum Cisco IOS-XE version required for each. A part number that is absent, or listed against a version newer than the one running, will not come up — and that has nothing to do with whether the optic is genuine. Cisco documents a case where a GLC-T failed on a Catalyst 3560CX until the switch was upgraded to IOS 15.2(5)E.
Run this before ordering, not after delivery. A module that requires a newer release turns a simple purchase into a software upgrade project, and that is much cheaper to discover in advance.
Check three: the far end
Two modules can each be supported on their own switch and still fail to link — different reach classes, different wavelength plans, one expecting FEC and the other not. Cisco publishes a second tool for this, the Optics-to-Optics Interoperability Matrix at tmgmatrix.cisco.com/iop.
Cisco's own description of why it exists is unusually candid: customers and sales previously had to "determine what transceivers can interoperate with which transceiver, by looking at multiple sources and asking various engineering groups," a process it describes as "time consuming and often leads to frustration."
The far-end check matters most on links to non-Cisco equipment, on BiDi modules where the two ends are deliberately different part numbers, and on 25G where FEC requirements differ by cable length.
Speed mixing: what is actually documented
| Question | Answer |
|---|---|
| 1G SFP in an SFP+ port | Generally yes, platform-dependent. The C9300-NM-8X is documented as a "10G/1G" module, and Cisco sells SFP-1G-SX, SFP-1G-LH and SFP-1G-T-X explicitly "for 10GE ports". Note the module's ports default to 10G. |
| 10G SFP+ in a 1G port | Not documented either way. The C9300-NM-4G is specified only for standard 1G SFP modules. Check the matrix rather than assuming. |
| 10G in an SFP28 port | Yes. The C9300X-NM-8Y and the C9300X-24Y downlinks are specified as 25/10/1 GE. |
| 25G-only module in an SFP+ port | Not documented. Cisco documents dual-rate optics instead — see below. |
| 40G QSFP+ in a 100G QSFP28 port | Implied but not stated directly. The C9300X-NM-2C and NM-4C are specified as "40/100 GE", which covers the practical case. |
Dual-rate optics are the documented answer for 10G/25G
Rather than mixing single-rate modules across port speeds, Cisco's stated approach is dual-rate parts: "Cisco's 25G portfolio provides full backward compatibility with 10G with dual-rate optics. These optics will automatically negotiate with the far-end device to the highest speed supported."
The relevant part numbers carry the dual rate in the name — SFP-10/25G-CSR-S, SFP-10/25G-LR-S, SFP-10/25G-LR-I, SFP-10/25G-BXD-I and BXU-I. Buying these instead of 25G-only modules removes the question entirely and future-proofs a link that starts at 10G.
Module and slot restrictions on the Catalyst 9300
| Module | Ports | Fits |
|---|---|---|
| C9300-NM-4G | 4 × 1G SFP | Modular C9300 |
| C9300-NM-8X | 8 × 10G SFP+ (ports default to 10G) | Modular C9300 |
| C9300-NM-2Q | 2 × 40G QSFP+ | Modular C9300 |
| C9300-NM-2Y | 2 × 25G SFP28 | Modular C9300 |
| C9300X-NM-2C | 2 × 40/100G | C9300X only |
| C9300X-NM-4C | 4 × 40/100G | Only C9300X-48HX, 48TX, 24Y |
| C9300X-NM-8Y | 8 × 25/10/1G | Only C9300X-24Y, 48HX, 48TX |
Two operational notes from Cisco's installation guide worth carrying into a change window. A module must be present in the uplink slot at all times. And interfaces can go error-disabled when a module with fibre attached is inserted or removed — so plan a hot swap accordingly rather than discovering it during a maintenance window.
Digital optical monitoring
DOM reports live optical transmit and receive power, temperature, laser bias current and supply voltage against warning and alarm thresholds, per the SFF-8472 multi-source agreement. It is the single most useful diagnostic on a fibre link and it is not universal — Cisco states plainly that "DOM is not integrated into all transceivers, and the minimum Cisco IOS XE version required varies depending on the switch hosting the transceiver."
Two practical markers. On 1G modules a trailing D in the part number means DOM is present — GLC-SX-MMD, GLC-LH-SMD, GLC-EX-SMD, GLC-ZX-SMD. The copper 1000BASE-T SFPs — GLC-T, GLC-TE, SFP-GE-T, SFP-1G-T-X — have no DOM at all, which is worth knowing before you specify copper SFPs for a link you will need to troubleshoot remotely.
show interfaces <id> transceiver detail
show interfaces transceiver dom-supported-list
show idprom interface <id> detail
Reading the output: each parameter is shown against high alarm, high warning, low warning and low alarm thresholds. Cisco's guidance is that receive power sitting below the low warning threshold most of the time indicates a problem with the transceiver at the other end of the link — the diagnostic points across, not down.
What to send a supplier for a clean answer
- The switch part number — not the family. Module support differs between a C9300-48P and a C9300X-48HX.
- The software version running on it.
- Which slot — uplink module or downlink port, and which module if applicable.
- The far-end device and its optic, if it is not Cisco.
- The distance and fibre type — OM3, OM4 or single-mode, and the actual run length.
Any supplier who can answer from that without asking follow-ups has actually checked. One who quotes a part number from the speed alone has not.
Send the platform and the distance, we'll send the part number
Switch model, software version, slot, fibre type and run length. We come back with modules that are supported on that platform at that release, the far-end match, and firm lead times — with the compatibility checked rather than assumed. Within one business day.
Frequently asked questions
How do I check whether a Cisco optic works in my switch?
Three checks. Cisco's Optics-to-Device Compatibility Matrix at tmgmatrix.cisco.com for the platform, show interfaces transceiver supported-list on the switch for the software version, and the Optics-to-Optics Interoperability Matrix for the far end.
Can I use a 1G SFP in a 10G SFP+ port?
Generally yes, and Cisco sells modules specifically for it — SFP-1G-SX, SFP-1G-LH and SFP-1G-T-X are described as being for 10GE ports. Support is platform-dependent, and on the C9300-NM-8X the ports default to 10G.
Can I use a 25G optic in a 10G port?
Cisco does not document that. What it does document is dual-rate 10/25G optics — SFP-10/25G-CSR-S, LR-S and others — which negotiate to the highest speed the far end supports. On a link that starts at 10G and may go to 25G, those are the correct part to buy.
Does my optic support DOM?
Not all do. On 1G modules a trailing D in the part number indicates DOM. Copper 1000BASE-T SFPs have none. Support also depends on the software version of the hosting switch — check with show interfaces transceiver dom-supported-list.
Where are the per-platform transceiver compatibility tables?
Cisco consolidated them into the online matrix tools. The old compatibility-tables page now carries only the user manuals for the two matrices, even though some release notes still link to it.
Can I hot-swap an uplink module?
Cisco warns that interfaces may go error-disabled when a network module with fibre attached is inserted or removed, and requires that a module be present in the uplink slot at all times. Plan the swap rather than treating it as transparent.
Related
- Cisco optics part numbers decoded — what SR, LR, ER and the suffixes mean
- Unsupported transceiver errors — telling a fake from an incompatibility
- Nexus 9300 refresh — QSFP-DD backward compatibility and breakout
- Reading a Catalyst part number
- Optic modules and transceivers · Enterprise switches · Cisco stock and lead times
Sources
- Cisco Optics-to-Device Compatibility Matrix · Optics-to-Optics Interoperability Matrix · user manual
- Cisco — installing a network module on Catalyst 9300
- Cisco — troubleshooting fibre links on Catalyst 9000 (DOM, supported-list)
- Cisco — 25G and 100G white paper (dual-rate optics)
- Cisco — 25G SFP28 data sheet · 1G SFP data sheet