The Cisco ISR 4321 is the only ISR 4000 platform built without an SM-X service-module slot, pairing 2 onboard Gigabit Ethernet ports with 2 NIM slots and a single 4 GB DRAM pool. Default IPv4 forwarding runs at 50 Mbps, rising to 100 Mbps once FL-4320-PERF-K9 is installed. On 7 November 2023, Cisco stopped selling it new, and support runs to 30 November 2028.
Ports, slots and memory
Port and slot counts on the ISR 4321 are the smallest in the ISR 4000 series: 2 onboard Gigabit Ethernet ports, 2 NIM slots, and zero SM-X service-module slots. Memory is a single combined 4 GB pool of DRAM rather than the split control-plane and data-plane arrangement used on the larger platforms in the series, and onboard flash is 4 GB. Anyone replacing an older branch router should count NIM requirements before anything else, because two slots is the hard ceiling on this chassis and there is no service-module bay to absorb the overflow.
| Specification | ISR 4321 |
|---|---|
| Onboard Gigabit Ethernet | 2 |
| NIM slots | 2 |
| SM-X slots | 0 |
| Default DRAM | 4 GB combined |
| Default flash | 4 GB |
| Default IPv4 forwarding throughput | 50 Mbps |
| With Performance licence | 100 Mbps (FL-4320-PERF-K9) |
| With Boost licence | 1.6 Gbps @ IMIX (FL-4320-BOOST-K9) |
Throughput and the performance licence
A Cisco ISR 4321 forwards IPv4 traffic at 50 Mbps by default, 100 Mbps with the FL-4320-PERF-K9 performance licence, and 1.6 Gbps at IMIX with FL-4320-BOOST-K9. Cisco publishes the Boost figure in Table 8 of the ISR 4000 data sheet and labels the row "IPv4 Forwarding Throughput", so a buyer holding this page beside the data sheet is reading the same measurement rather than a marketing number.
Nothing on the outside of the chassis tells you which tier a used unit carries. Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, so the tier a unit arrives with is the tier it keeps — a 50 Mbps ISR 4321 cannot be lifted to 100 Mbps later by buying FL-4320-PERF-K9 new from Cisco. Sizing therefore has to happen before the order rather than after delivery, and the question that matters is not which router the site needs but which licensed forwarding rate the circuit needs.
For most ISR 4321 deployments that question resolves quickly, because the platform exists for sub-100 Mbps branch circuits. A site on a 50 Mbps service runs on a bare unit. A site on a 100 Mbps service needs FL-4320-PERF-K9 already installed. Anything above 100 Mbps is either a Boost-licensed 4321 at 1.6 Gbps at IMIX or, more often, the wrong platform for the job.
Which software bundle you need
Software bundle suffixes on the ISR 4321 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, V, VSEC, AX and AXV. AX already contains Security, so buying a separate Security entitlement for an AX unit pays twice for the same capability — the most common and most expensive mistake made when ordering these routers.
| Suffix | Bundle | Cisco part number |
|---|---|---|
| Base | IP Base only | ISR4321/K9 |
| V | Unified Communications | ISR4321-V/K9 |
| VSEC | Security plus Unified Communications | ISR4321-VSEC/K9 |
| AX | Application Experience, includes Security | ISR4321-AX/K9 |
| AXV | Application Experience plus Unified Communications | ISR4321-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 4321 is 2 NIM slots, and there is no SM-X service-module bay on this platform at all. That is the clearest dividing line between the 4321 and every other router in the series: a requirement that includes a service module cannot be satisfied on this chassis in any configuration, and the smallest platform that can satisfy it is the ISR 4331 with its single SM-X slot. Specific NIM part numbers are confirmed against your requirement before the order rather than listed here.
Where the ISR 4321 sits in the series
The step from the ISR 4321 to the ISR 4331 buys one SM-X service-module slot, a third Gigabit Ethernet port, and double the default forwarding rate.
| Specification | ISR 4321 | ISR 4331 |
|---|---|---|
| Onboard Gigabit Ethernet | 2 | 3 |
| NIM slots | 2 | 2 |
| SM-X slots | 0 | 1 |
| Default DRAM | 4 GB combined | 4 GB combined |
| Default IPv4 forwarding throughput | 50 Mbps | 100 Mbps |
| With Performance licence | 100 Mbps | 300 Mbps |
| With Boost licence | 1.6 Gbps @ IMIX | 1.8 Gbps @ IMIX |
Both platforms carry the same single 4 GB pool of DRAM and the same 2 NIM slots, so neither memory nor NIM capacity is a reason to step up. The reasons are the SM-X slot, the third Gigabit Ethernet port, and the move from 50 Mbps to 100 Mbps before any licence is applied, widening to 100 Mbps against 300 Mbps once the performance licence is installed. At the Boost tier the two platforms are close, 1.6 Gbps at IMIX here against 1.8 Gbps at IMIX on the ISR 4331, which is worth knowing before paying a premium for the larger chassis on throughput grounds alone. A site that needs three NIM slots or two service-module bays skips both and looks at the ISR 4351. The full range of refurbished Cisco switches and routers we hold sits alongside these platforms.
Lifecycle and support horizon
Cisco stopped accepting orders for the ISR 4321 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, running in branch offices 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 4321 is specified, because it means the throughput tier is a property of the individual unit rather than something that can be corrected after it lands on the rack.
Condition, testing and what ships with the router
Every ISR 4321 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 4321?
A Cisco ISR 4321 forwards IPv4 traffic at 50 Mbps by default. With the FL-4320-PERF-K9 performance licence installed it forwards at 100 Mbps, and with FL-4320-BOOST-K9 it reaches 1.6 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 depends entirely on which licence that unit carries.
Does a refurbished ISR 4321 come with a performance licence?
Not unless the listing states otherwise. Software licences and subscriptions are supplied separately from the hardware, and Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, which means a missing tier cannot be bought new from Cisco afterwards. Tell us the forwarding rate your circuit needs before ordering and we will confirm in writing what the specific unit being shipped carries.
What is the difference between ISR4321/K9 and ISR4321-AX/K9?
The suffix identifies the software bundle the router was originally built with. ISR4321/K9 is the IP Base build, while ISR4321-AX/K9 is the Application Experience bundle, which already contains Security. Because AX includes Security, buying a separate Security entitlement for an AX unit pays twice for the same capability. Neither suffix says anything about which throughput licence is installed on a used unit today.
Is the Cisco ISR 4321 end of life?
Cisco stopped accepting orders for the ISR 4321 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.
Can the ISR 4321 take a service module?
No. The ISR 4321 has zero SM-X service-module slots, and it is the only platform in the ISR 4000 series with none at all. It offers 2 NIM slots and nothing beyond them. A requirement that includes a service module cannot be met on this chassis in any configuration, and the smallest platform that can meet it is the ISR 4331, which carries one SM-X slot alongside two NIM slots.
How much memory does an ISR 4321 have?
An ISR 4321 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. Sources quoting 8 GB for this platform have doubled a single pool. The split arrangement belongs to the larger 4400-series routers, which carry 4 GB for the control plane plus 2 GB for the data plane and 8 GB of flash.
