Module density rather than raw bandwidth is the case for the Cisco ISR 4351: 3 NIM slots plus 2 SM-X service-module slots, against 2 and 1 on the ISR 4331 below it. It keeps the same 3 onboard Gigabit Ethernet ports and the same single 4 GB DRAM pool. Default IPv4 forwarding runs at 200 Mbps, and 400 Mbps once a performance licence is installed. Cisco stopped selling the platform new on 7 November 2023, and supports it to 30 November 2028.
Ports, slots and memory
Module density is what distinguishes the ISR 4351: 3 NIM slots and 2 SM-X service-module slots, against 2 NIM and 1 SM-X on the ISR 4331 below it. Onboard connectivity does not change between the two platforms — both carry 3 Gigabit Ethernet ports — and neither does memory, which is a single combined 4 GB pool of DRAM with 4 GB of onboard flash on both. A branch that outgrew a 4331 almost always outgrew its slots rather than its ports, and that is the requirement this platform exists to serve.
| Specification | ISR 4351 |
|---|---|
| Onboard Gigabit Ethernet | 3 |
| NIM slots | 3 |
| SM-X slots | 2 |
| ISC slot | 1 |
| Default DRAM | 4 GB combined |
| Default flash | 4 GB |
| Default IPv4 forwarding throughput | 200 Mbps |
| With Performance licence | 400 Mbps (FL-4350-PERF-K9) |
| With Boost licence | 2 Gbps @ IMIX (FL-4350-BOOST-K9) |
Throughput and the performance licence
A Cisco ISR 4351 forwards IPv4 traffic at 200 Mbps by default, 400 Mbps with the FL-4350-PERF-K9 performance licence, and 2 Gbps at IMIX with FL-4350-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 a buyer should compare against rather than a headline capacity claim.
The gap between a 4331 and a 4351 at the licensed tier is 300 Mbps against 400 Mbps — a third more, not a different class of router. Saying that plainly is more useful than pretending the 4351 is the bandwidth answer: this platform is chosen for its slots, and the licence tier still decides what the circuit can carry. A 4351 running without a performance licence forwards at 200 Mbps no matter how many service modules are installed in it.
Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, so the tier a used unit carries cannot be raised afterwards by buying the licence new. Nothing on the outside of the chassis indicates which tier is installed, which is why the forwarding rate the circuit needs should be settled before the order rather than discovered during commissioning.
Which software bundle you need
Software bundle suffixes on the ISR 4351 describe the entitlement the router was originally built with, and they are a separate axis from the throughput licence. Five bundles are held for this platform: Base, SEC, V, VSEC and AXV. Where an AX build is encountered on this platform elsewhere, note that AX already contains Security — a separate Security entitlement layered on top of it pays twice for the same capability, which is worth knowing for anyone arriving here after comparing AX-suffixed 4331 units.
| Suffix | Bundle | Cisco part number |
|---|---|---|
| Base | IP Base only | ISR4351/K9 |
| SEC | Security bundle | ISR4351-SEC/K9 |
| V | Unified Communications | ISR4351-V/K9 |
| VSEC | Security plus Unified Communications | ISR4351-VSEC/K9 |
| AXV | Application Experience plus Unified Communications | ISR4351-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 4351 is 3 NIM slots, 2 SM-X service-module slots and 1 ISC slot, which is the highest slot count in the 4300 range. Two service-module bays is the specification that a 4331 cannot match and, more surprisingly, that the larger ISR 4431 cannot match either — the 4431 has none. Anyone consolidating two service modules into one chassis is choosing between this platform and the ISR 4451-X, not between this platform and the next number up. Specific NIM and SM-X part numbers are confirmed against your requirement before the order rather than listed here.
Where the ISR 4351 sits in the series
The ISR 4431 forwards more than twice as fast as the 4351 by default, but it has no SM-X slot at all — a 4351 running two service modules cannot be replaced by a 4431 without losing them.
| Specification | ISR 4331 | ISR 4351 | ISR 4431 |
|---|---|---|---|
| Onboard Gigabit Ethernet | 3 | 3 | 4 |
| NIM slots | 2 | 3 | 3 |
| SM-X slots | 1 | 2 | 0 |
| Default DRAM | 4 GB combined | 4 GB combined | 4 GB control plus 2 GB data |
| Default flash | 4 GB | 4 GB | 8 GB |
| Default IPv4 forwarding throughput | 100 Mbps | 200 Mbps | 500 Mbps |
| With Performance licence | 300 Mbps | 400 Mbps | 1 Gbps |
| With Boost licence | 1.8 Gbps @ IMIX | 2 Gbps @ IMIX | 3.4 Gbps @ IMIX |
Stepping down to the ISR 4331 costs a NIM slot and a service-module bay and halves default forwarding from 200 Mbps to 100 Mbps, while the licensed tier falls from 400 Mbps to 300 Mbps. Stepping up to the ISR 4431 is not a straight upgrade: default forwarding rises from 200 Mbps to 500 Mbps and the licensed tier from 400 Mbps to 1 Gbps, memory changes to a split 4 GB control-plane and 2 GB data-plane arrangement with 8 GB of flash, but every SM-X slot disappears. Sites needing both gigabit-class forwarding and service modules skip the 4431 entirely. The smallest platform in the range, the ISR 4321, sits two steps below with no service-module support at all, and the wider catalogue of refurbished Cisco switches and routers covers the switching side of the same build.
Lifecycle and support horizon
Cisco stopped accepting orders for the ISR 4351 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, carrying 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 4351 is specified, because it means the throughput tier travels with the individual unit and cannot be corrected once the router is in service.
Condition, testing and what ships with the router
Every ISR 4351 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 4351?
A Cisco ISR 4351 forwards IPv4 traffic at 200 Mbps by default. With the FL-4350-PERF-K9 performance licence installed it forwards at 400 Mbps, and with FL-4350-BOOST-K9 it reaches 2 Gbps at IMIX, the figure Cisco publishes in Table 8 of the ISR 4000 data sheet. Installing service modules does not change any of those rates, because the forwarding tier is set by the licence rather than by what occupies the slots.
Does a refurbished ISR 4351 come with a performance licence?
Licensing is handled separately from the chassis. An unlicensed ISR 4351 forwards at 200 Mbps, and the step to 400 Mbps depends on FL-4350-PERF-K9 already sitting on that specific unit. Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, so the tier cannot be added later. Tell us the rate your circuit needs and the answer goes in writing before you order.
What is the difference between ISR4351/K9 and ISR4351-VSEC/K9?
The suffix identifies the software bundle the router was originally built with. ISR4351/K9 is the IP Base build, while ISR4351-VSEC/K9 combines the Security bundle with Unified Communications. SEC alone carries Security, V alone carries Unified Communications, and AXV is Application Experience with Unified Communications. Application Experience builds already contain Security, so layering a separate Security entitlement on one pays twice for the same capability.
Is the Cisco ISR 4351 end of life?
Cisco stopped accepting orders for the ISR 4351 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.
What is the difference between an ISR 4331 and an ISR 4351?
Slot count is the real difference. Both platforms carry 3 onboard Gigabit Ethernet ports, but the ISR 4351 has 3 NIM slots and 2 SM-X service-module slots against 2 NIM and 1 SM-X on the ISR 4331. Default forwarding also doubles from 100 Mbps to 200 Mbps, and the licensed tier moves from 300 Mbps to 400 Mbps, but the module capacity is the reason most buyers step up.
Can an ISR 4431 replace an ISR 4351?
On throughput, comfortably. On modules, no. An ISR 4431 forwards at 500 Mbps by default against 200 Mbps on the ISR 4351, but it has zero SM-X service-module slots where the 4351 has two, so any service module fitted to a 4351 has nowhere to go in a 4431. Both platforms carry 3 NIM slots, so NIM-based builds do migrate cleanly.
