Hosting a service module is what separates the Cisco ISR 4331 from the platform below it: 1 SM-X slot, alongside 3 onboard Gigabit Ethernet ports, 2 NIM slots and a 4 GB DRAM pool. Default IPv4 forwarding runs at 100 Mbps, rising to 300 Mbps with FL-4330-PERF-K9 and 1.8 Gbps at IMIX with Boost, which is the platform ceiling. On 7 November 2023, Cisco stopped selling the platform new, and support runs to 30 November 2028.
Ports, slots and memory
Port and slot counts on the ISR 4331 are 3 onboard Gigabit Ethernet ports, 2 NIM slots, 1 SM-X service-module slot and 1 ISC slot. Memory is a single combined 4 GB pool of DRAM rather than the split control-plane and data-plane arrangement the 4400-series routers use, and onboard flash is 4 GB. The SM-X slot is the specification that defines this platform commercially: it is the smallest ISR 4000 that can host a service module, and it is the reason most branch refreshes that outgrow a 4321 land here rather than further up the range.
| Specification | ISR 4331 |
|---|---|
| Onboard Gigabit Ethernet | 3 |
| NIM slots | 2 |
| SM-X slots | 1 |
| ISC slot | 1 |
| Default DRAM | 4 GB combined |
| Default flash | 4 GB |
| Default IPv4 forwarding throughput | 100 Mbps |
| With Performance licence | 300 Mbps (FL-4330-PERF-K9) |
| With Boost licence | 1.8 Gbps @ IMIX (FL-4330-BOOST-K9) |
Throughput and the performance licence
A Cisco ISR 4331 forwards IPv4 traffic at 100 Mbps by default, 300 Mbps with the FL-4330-PERF-K9 performance licence, and 1.8 Gbps at IMIX with FL-4330-BOOST-K9. Cisco publishes the Boost figure in Table 8 of the ISR 4000 data sheet and labels the row "IPv4 Forwarding Throughput", which is the row to compare against when a data sheet and a listing appear to disagree.
A buyer sizing for a 200 Mbps circuit who orders a bare ISR 4331 has bought a router that cannot carry it, and a buyer who assumes that a 4331 simply does 300 Mbps has bought the wrong tier. Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, so the gap cannot be closed after delivery at any price. The throughput tier is a property of the individual unit, in the same way the serial number is, and it is not visible from the outside of the chassis.
That makes the order of operations unusual for a hardware purchase. The circuit rate decides the licence tier, the licence tier decides which units are candidates, and only then does the chassis matter. A site running a 100 Mbps handoff is served by a bare unit. A site on 200 or 300 Mbps needs FL-4330-PERF-K9 already installed on the unit it receives. A site that expects to grow past 300 Mbps needs a Boost-licensed unit at 1.8 Gbps at IMIX, or a larger platform entirely.
Which software bundle you need
Software bundle suffixes on the ISR 4331 describe the entitlement the router was originally built with, and they are a separate axis from the throughput licence. This is the one platform in the series where the full set of six bundles is held: Base, SEC, V, VSEC, AX and AXV. AX already contains Security, so buying a separate SEC entitlement for an AX unit pays twice for the same capability — the most common and most expensive mistake made when ordering these routers, and the reason the fact is stated on the option itself rather than buried in a comparison further down.
| Suffix | Bundle | Cisco part number |
|---|---|---|
| Base | IP Base only | ISR4331/K9 |
| SEC | Security bundle | ISR4331-SEC/K9 |
| V | Unified Communications | ISR4331-V/K9 |
| VSEC | Security plus Unified Communications | ISR4331-VSEC/K9 |
| AX | Application Experience, includes Security | ISR4331-AX/K9 |
| AXV | Application Experience plus Unified Communications | ISR4331-AXV/K9 |
The suffix describes the software bundle the unit was originally built with. It is not a promise that a used unit still carries an installed throughput licence — that is confirmed per unit on the quote.
Module slots: what this router can take
Module capacity on the ISR 4331 is 2 NIM slots, 1 SM-X service-module slot and 1 ISC slot. That single SM-X bay is what separates this platform from the ISR 4321 below it, which has none, and it is also where the 4331 stops: a site needing two service modules has to move to an ISR 4351, which carries 2 SM-X slots and a third NIM slot. Specific NIM and SM-X part numbers are confirmed against your requirement before the order rather than listed here.
Where the ISR 4331 sits in the series
The ISR 4351 keeps the same three Gigabit Ethernet ports as the 4331 but adds a third NIM slot and a second SM-X slot, and lifts default forwarding from 100 Mbps to 200 Mbps.
| Specification | ISR 4321 | ISR 4331 | ISR 4351 |
|---|---|---|---|
| Onboard Gigabit Ethernet | 2 | 3 | 3 |
| NIM slots | 2 | 2 | 3 |
| SM-X slots | 0 | 1 | 2 |
| Default flash | 4 GB | 4 GB | 4 GB |
| Default IPv4 forwarding throughput | 50 Mbps | 100 Mbps | 200 Mbps |
| With Performance licence | 100 Mbps | 300 Mbps | 400 Mbps |
| With Boost licence | 1.6 Gbps @ IMIX | 1.8 Gbps @ IMIX | 2 Gbps @ IMIX |
Stepping down to the ISR 4321 costs the SM-X slot entirely, drops one Gigabit Ethernet port, and halves default forwarding from 100 Mbps to 50 Mbps. Stepping up to the ISR 4351 keeps the port count identical and buys module density instead: a third NIM slot and a second SM-X slot, with default forwarding doubling to 200 Mbps and the licensed tier moving from 300 Mbps to 400 Mbps. All three platforms carry the same 4 GB of flash and the same single 4 GB pool of DRAM, so memory never decides between them. Sites that need appreciably more than 400 Mbps leave the 4300 range altogether for the ISR 4431, which forwards at 500 Mbps before any licence is applied but gives up SM-X support to do it. All of these platforms sit within the wider ISR 4000 series, and the range of refurbished Cisco switches and routers we hold covers the switching side of the same branch build.
Lifecycle and support horizon
Cisco stopped accepting orders for the ISR 4331 on 7 November 2023 and supports the platform until 30 November 2028, both dates published in end-of-life notice EOL14896. Those two facts belong together and neither works alone: end-of-sale on its own reads as obsolete, and a support horizon on its own does not explain why the hardware is affordable. Read together they describe the window in which this router is bought second-hand — a platform Cisco no longer sells but still supports, holding branch sites that do not need a refresh before that horizon closes.
Cisco's notice on the performance and Boost licences for this platform records that no replacement product is available. That is the sentence that should govern how a used ISR 4331 is specified, because it means the throughput tier is a property of the individual unit rather than a gap that can be filled after the router is racked.
Condition, testing and what ships with the router
Every ISR 4331 we ship is refurbished enterprise networking hardware, tested when it arrives and tested again before it leaves. The process is fixed: inbound test on receipt, disassemble, clean, inventory, pick, assemble, configure, and a final outbound test before the unit is packed. Routers ship with a 2-year Enterasource warranty and free FedEx Ground delivery to the lower 48 states.
Software licences and subscriptions are not included unless the listing states otherwise. Cisco's throughput performance and Boost licences for this platform are end-of-sale with no replacement product available, so a licence cannot be purchased new from Cisco after the fact. Tell us the forwarding rate your circuit needs before you order and we will confirm, in writing, what the specific unit we are shipping carries.
Frequently Asked Questions
What is the throughput of a Cisco ISR 4331?
A Cisco ISR 4331 forwards IPv4 traffic at 100 Mbps by default. With the FL-4330-PERF-K9 performance licence installed it forwards at 300 Mbps, and with FL-4330-BOOST-K9 it reaches 1.8 Gbps at IMIX, the figure Cisco publishes in Table 8 of the ISR 4000 data sheet. All three tiers run on identical hardware, so the rate a given unit delivers is decided by the licence that unit carries rather than by the chassis.
Does a refurbished ISR 4331 come with a performance licence?
Only where the listing says so. A bare ISR 4331 forwards at 100 Mbps, and reaching 300 Mbps depends on FL-4330-PERF-K9 already being present on the individual unit that ships. Because Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, that tier cannot be bought new afterwards. Tell us the rate your circuit needs and we will confirm in writing what the unit carries.
What is the difference between ISR4331/K9 and ISR4331-SEC/K9?
The suffix identifies the software bundle the router was originally built with. ISR4331/K9 is the IP Base build; ISR4331-SEC/K9 adds the Security bundle. V adds Unified Communications, VSEC combines Security and Unified Communications, AX is Application Experience and already contains Security, and AXV adds Unified Communications to AX. Because AX includes Security, a separate SEC entitlement on an AX unit pays twice for the same capability.
Is the Cisco ISR 4331 end of life?
Cisco stopped accepting orders for the ISR 4331 on 7 November 2023 and supports the platform until 30 November 2028, both dates published in end-of-life notice EOL14896. The platform is therefore end-of-sale but still inside its support horizon, which is precisely the window in which these routers are bought on the secondary market for branch sites that do not need a hardware refresh before that horizon closes.
How many modules can an ISR 4331 take?
An ISR 4331 has 2 NIM slots, 1 SM-X service-module slot and 1 ISC slot. That is one service-module bay, which is one more than the ISR 4321 offers and one fewer than the ISR 4351, and the 4351 also carries a third NIM slot. A build that needs two service modules cannot be delivered on a 4331 in any configuration, so slot counting has to happen before the platform is chosen.
How much memory does an ISR 4331 have?
An ISR 4331 carries a single combined 4 GB pool of DRAM rather than a split control-plane and data-plane pair, together with 4 GB of onboard flash. The ISR 4431 and ISR 4451-X are the split platforms, carrying 4 GB for the control plane plus 2 GB for the data plane and 8 GB of flash; the 4321, 4331 and 4351 are not. Sources quoting 8 GB on a 4331 have doubled a single pool.
