Gigabit-class forwarding and service-module support meet on one platform in this range: the Cisco ISR 4451-X. It has 4 onboard Gigabit Ethernet ports, 3 NIM slots, 2 SM-X slots, 4 GB of control-plane DRAM plus 2 GB for the data plane, and 8 GB of flash. Default IPv4 forwarding runs at 1 Gbps, which is the licensed ceiling of the smaller ISR 4431, and 2 Gbps once a performance licence is installed. Cisco no longer sells it new, and the secondary market is where it is bought.
Ports, slots and memory
Service-module capacity is what sets the ISR 4451-X apart from the rest of the 4400 range: 2 SM-X slots alongside 3 NIM slots and 1 ISC slot, where the ISR 4431 has no SM-X bay at all. Onboard connectivity is 4 Gigabit Ethernet ports, the same count the 4431 carries. Memory follows the 4400-series pattern rather than the 4300-series one, with 4 GB of control-plane DRAM plus 2 GB for the data plane and 8 GB of onboard flash, so a build moving up from a 4431 keeps its memory and flash profile and gains only slots and speed.
| Specification | ISR 4451-X |
|---|---|
| Onboard Gigabit Ethernet | 4 |
| NIM slots | 3 |
| SM-X slots | 2 |
| ISC slot | 1 |
| Default DRAM | 4 GB control plus 2 GB data |
| Default flash | 8 GB |
| Default IPv4 forwarding throughput | 1 Gbps |
| With Performance licence | 2 Gbps (FL-44-PERF-K9) |
| With Boost licence | 3.8 Gbps @ IMIX (FL-44-BOOST-K9) |
Throughput and the performance licence
A Cisco ISR 4451-X forwards IPv4 traffic at 1 Gbps by default, 2 Gbps with the FL-44-PERF-K9 performance licence, and 3.8 Gbps at IMIX with FL-44-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 worth checking whenever two listings appear to disagree.
A bare 4451-X already matches a fully licensed ISR 4431, since both forward at 1 Gbps, and that is the comparison worth running before paying for a performance tier on the smaller platform. 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 is the tier it will always have. Nothing on the outside of the chassis reveals which tier is installed.
Cisco lists the same performance licence part number, FL-44-PERF-K9, for both the ISR 4431 and the ISR 4451-X, while the Boost part numbers differ — FL-44-BOOST-K9 on this platform and FL-4430-BOOST-K9 on the 4431. Those are the part numbers as Cisco lists them, and a shared part number should not be read as a statement that an entitlement moves between the two platforms.
Which software bundle you need
Software bundle suffixes on the ISR 4451-X 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, AX and AXV. This is the one platform in the range where no VSEC build is held, which makes the AX entry unusually important: AX already contains Security, so besides SEC itself it is the only security-carrying bundle available here, and layering a separate Security entitlement on top of an AX unit pays twice for the same capability.
| Suffix | Bundle | Cisco part number |
|---|---|---|
| Base | IP Base only | ISR4451-X/K9 |
| SEC | Security bundle | ISR4451-X-SEC/K9 |
| V | Unified Communications | ISR4451-X-V/K9 |
| AX | Application Experience, includes Security | ISR4451-X-AX/K9 |
| AXV | Application Experience plus Unified Communications | ISR4451-X-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 4451-X is 3 NIM slots, 2 SM-X service-module slots and 1 ISC slot. That combination is the reason this platform exists: it is the only router in the 4400 range that pairs gigabit-class default forwarding with service-module support, and a site needing both has no alternative within the series. Below it, the ISR 4431 forwards fast but hosts no service module; the ISR 4351 hosts two but forwards at 200 Mbps until licensed. Specific NIM and SM-X part numbers are confirmed against your requirement before the order rather than listed here.
Where the ISR 4451-X sits in the series
The two 4400-series platforms share four Gigabit Ethernet ports, three NIM slots, the same memory and flash, and the same performance-licence part number — the 4451-X doubles the forwarding rate at every tier and adds the two SM-X slots the 4431 does not have.
| Specification | ISR 4431 | ISR 4451-X |
|---|---|---|
| Onboard Gigabit Ethernet | 4 | 4 |
| NIM slots | 3 | 3 |
| SM-X slots | 0 | 2 |
| Default DRAM | 4 GB control plus 2 GB data | 4 GB control plus 2 GB data |
| Default flash | 8 GB | 8 GB |
| Default IPv4 forwarding throughput | 500 Mbps | 1 Gbps |
| With Performance licence | 1 Gbps (FL-44-PERF-K9) | 2 Gbps (FL-44-PERF-K9) |
| With Boost licence | 3.4 Gbps @ IMIX (FL-4430-BOOST-K9) | 3.8 Gbps @ IMIX (FL-44-BOOST-K9) |
Memory, flash, port count and NIM count are identical across the two platforms, so the decision reduces to two questions: does the site need more than 1 Gbps of licensed forwarding, and does it need a service module. Either answer of yes points here. Stepping down to the ISR 4431 halves forwarding at the default and licensed tiers and removes SM-X support entirely, while the Boost figures stay close at 3.4 Gbps against 3.8 Gbps at IMIX. Module-heavy branch builds that do not need gigabit forwarding are usually better served by the ISR 4351, which also carries 2 SM-X slots, and smaller sites by the ISR 4331. The wider catalogue of refurbished Cisco switches and routers covers the switching side of the same build.
Lifecycle and support horizon
Cisco no longer sells the ISR 4451-X new, which is why the secondary market is where this platform is bought. End-of-sale is not the same as end-of-support, and confusing the two costs buyers money in both directions: a router nobody sells new can still be a supported production asset, and the discount that comes with end-of-sale status is precisely what makes a refurbished unit sensible at the top of this range. The exact milestone dates published for this platform are confirmed on request rather than quoted here, because the applicable Cisco notice covers a named subset of the 4400-series part numbers and we do not publish a date we have not matched to the exact part number being sold.
Cisco's notice on the performance and Boost licences records that no replacement product is available. That governs how a used ISR 4451-X should be specified, because the throughput tier travels with the individual unit and is not something that can be added once the router is carrying traffic.
Condition, testing and what ships with the router
Every ISR 4451-X 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 4451-X?
The ISR 4451, sold by Cisco as the ISR 4451-X, forwards IPv4 traffic at 1 Gbps by default. With the FL-44-PERF-K9 performance licence installed it forwards at 2 Gbps, and with FL-44-BOOST-K9 it reaches 3.8 Gbps at IMIX, the figure Cisco publishes in Table 8 of the ISR 4000 data sheet. A bare unit therefore matches the licensed ceiling of an ISR 4431 before any licence is bought.
Does a refurbished ISR 4451-X come with a performance licence?
Hardware and licensing are separate items, so assume not unless the listing states otherwise. A bare ISR 4451-X already forwards at 1 Gbps, while 2 Gbps requires FL-44-PERF-K9 on the unit itself. Cisco's performance and Boost licences for this platform are end-of-sale with no replacement product available, which means the tier cannot be added after delivery. We confirm in writing what the shipping unit carries.
What is the difference between ISR4451-X/K9 and ISR4451-X-AX/K9?
The suffix identifies the software bundle the router was originally built with. ISR4451-X/K9 is the IP Base build, while ISR4451-X-AX/K9 is the Application Experience bundle, which already contains Security. Adding a separate Security entitlement to an AX unit therefore pays twice for the same capability. On this platform there is no VSEC build, so AX and SEC are the two security-carrying bundles available.
Is the Cisco ISR 4451-X end of life?
Cisco no longer sells the ISR 4451-X new, which is why it is sourced on the secondary market. End-of-sale status is separate from end-of-support status, and a platform can sit well inside its support horizon while no longer being orderable from the manufacturer. The exact milestone dates that apply to this specific part number are confirmed on request rather than published here, because the relevant Cisco notice covers a named subset of the 4400-series routers.
What is the difference between an ISR 4431 and an ISR 4451-X?
Ports, NIM slots, memory and flash are identical: 4 onboard Gigabit Ethernet ports, 3 NIM slots, 4 GB of control-plane DRAM plus 2 GB for the data plane, and 8 GB of flash on both. The ISR 4451-X doubles forwarding at every tier, from 500 Mbps to 1 Gbps by default and from 1 Gbps to 2 Gbps when licensed, and it adds 2 SM-X service-module slots that the ISR 4431 does not have at all.
How many service modules can an ISR 4451-X take?
An ISR 4451-X has 2 SM-X service-module slots, 3 NIM slots and 1 ISC slot. That makes it the only platform in the 4400 range able to host a service module, since the ISR 4431 has none. Within the wider series only the ISR 4351 offers the same 2 SM-X slots, and it forwards considerably slower, so module-heavy builds that also need speed end up on this platform.
