Cisco 2921 vs ISR4331
The ISR 4331 is the modern IOS XE replacement for the end-of-life Cisco 2921 (ISR G2), delivering much higher throughput, far more memory, and Cisco SD-WAN in a compact 1RU chassis. Production 2921 routers should be migrated to the ISR 4331.
Cisco 2921 ISR G2
ISR Generation 2 branch router with three GE ports and four EHWIC slots on classic IOS 15.
- 3 onboard GE ports, 4 EHWIC slots, 1 ISM, 2 DSP slots
- 512 MB DRAM / 256 MB flash default (classic IOS 15)
- No native IOS XE or Cisco SD-WAN
- End-of-life; past last day of support
Cisco ISR 4331
Mid-range ISR 4000 branch router on IOS XE with NIM and SM slots and SD-WAN support.
- 3 onboard GE ports, 2 NIM slots, 1 SM slot
- 4 GB DRAM / 4 GB flash default
- Up to 2 Gbps aggregate throughput (license)
- Cisco IOS XE with Cisco SD-WAN support
Cisco 2921 ISR G2 vs Cisco ISR 4331: spec comparison
| Spec | Cisco 2921 ISR G2 | Cisco ISR 4331 |
|---|---|---|
| Onboard GE ports | 3 | 3 |
| Modular slots | 4 EHWIC + 1 ISM + 2 DSP | 2 NIM + 1 SM |
| Aggregate throughput | ~100 Mbps class (G2) | 100 Mbps to 2 Gbps (license) |
| Default DRAM | 512 MB | 4 GB |
| Default flash | 256 MB CF | 4 GB |
| Software | Cisco IOS 15 (classic) | Cisco IOS XE |
| Native SD-WAN | No | Yes (Cisco SD-WAN) |
| Voice / DSP | PVDM3 (2 DSP slots) | PVDM4 via NIM |
| Form factor | 2RU | 1RU |
| Lifecycle status | End-of-life | Mature but supported |
Choose Cisco 2921 ISR G2 if
Choose the 2921 only to keep an existing voice/branch site running on EHWIC and DSP modules you already own until a planned refresh; it is not a sensible new purchase.
Choose Cisco ISR 4331 if
Choose the ISR 4331 for refreshes: much higher throughput, 8x the DRAM, IOS XE, SM-slot expansion, and native Cisco SD-WAN in a smaller 1RU chassis.
Verdict
Migrate from the Cisco 2921 to the ISR 4331. The 2921 is end-of-life on classic IOS with limited memory, while the 4331 offers up to 2 Gbps, IOS XE, more memory, and SD-WAN in 1RU. Keep a 2921 only short-term where DSP/EHWIC modules and low bandwidth make immediate replacement uneconomical.
Frequently asked questions
What replaces the Cisco 2921 router?
The Cisco ISR 4331 is the modern replacement for the 2921, moving from classic IOS 15 with EHWIC/DSP slots to IOS XE, NIM and SM modules, much more memory, and Cisco SD-WAN support.
Is the ISR 4331 a good fit for a voice-enabled 2921?
Yes. The ISR 4331 supports voice via PVDM4 DSPs on NIM modules plus CUBE and SRST, covering 2921 voice use cases, though PVDM3 and EHWIC modules must be replaced with NIM equivalents.
Is the Cisco 2921 end of life?
Yes. The 2921 reached end-of-sale in the 2017 to 2018 window and is past its last day of support, so it should be replaced to keep a supported, patched software image.
How much more throughput does the ISR 4331 offer than the 2921?
The ISR 4331 scales up to 2 Gbps aggregate with a performance license, a large increase over the roughly 100 Mbps-class practical forwarding of the older 2921.
More ISR G comparisons
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