The Cisco Nexus 3408-S is an 8-slot, 4RU chassis delivering 25.6-Tbps switching capacity and up to 10 Bpps of packet forwarding. Utilizing a high-performance merchant silicon ASIC with 70 MB dynamic buffer allocation, it is positioned as a premier tier aggregation switch for 100G and 400G spine-and-leaf data center fabrics. It provides unparalleled port density flexibility to accommodate demanding NVMe over Fabrics, AI/ML clusters, and lossless Ethernet workloads.
System Architecture & Performance
When evaluating the Nexus 3408-S chassis, calculate the total required bisectional bandwidth for your spine-leaf architecture. With a 25.6-Tbps switching capacity, this 4RU chassis is optimized for high-density 100G and 400G aggregation. If your environment heavily relies on distributed storage (e.g., NVMe over Fabrics) or machine learning clusters, the massive 70 MB dynamic buffer allocation combined with 400G uplink capacity ensures lossless transmission during microbursts. Choose this 8-slot chassis over traditional fixed-form-factor switches when port radix flexibility and future 400G migration are critical to reducing the total number of fabric hops.
| Specification | Details |
|---|---|
| Chassis Form Factor | 4 RU, 8-slot chassis |
| Switching Capacity | 25.6-Tbps |
| Packet Forwarding Rate | 7.2 Bpps at ingress and 10 Bpps at egress |
| CPU and Memory | Four-core CPU, 32 GB of DRAM, 128 GB boot flash |
| Dynamic Buffer Allocation | 70 MB |
| Management Ports | One RJ45 and one SFP Management port, One RS-232 serial console, One USB port |
| Max 100G Ports | 128 |
| Max 400G Ports | 32 |
Line Expansion Modules (LEMs)
Selecting between the 100G (NXM-X16C) and 400G (NXM-X4D) LEMs depends entirely on your server-to-leaf and leaf-to-spine oversubscription ratios. Deploy the NXM-X16C (16x 100G) modules when aggregating existing 25G/50G server access switches or routing 100G native storage traffic. Conversely, the NXM-X4D (4x 400G) is strictly designed for high-capacity spine interconnects or next-generation 400G routing. If you need 50G or 25G breakouts to directly connect high-density compute nodes, factor in the exact breakout cable capabilities: the NXM-X16C supports 2x50G and 4x25G/10G splits, while the NXM-X4D handles 2×200/50G and 4×100/50/25/10G breakouts.
| Module Part Number | Description | Port Density | Breakout Support |
|---|---|---|---|
| NXM-X16C | Nexus 100G Line Expansion Module | 16 ports of QSFP28 | 2x50G, 4x25G/10G |
| NXM-X4D | Nexus 400G Line Expansion Module | 4 ports of QSFP-DD | 2×200/50G, 4×100/50/25G/10G |
| NXM-XBLNK | Nexus Blank Line Expansion Module | N/A | N/A |
Software Licensing Tiers
The software licensing tier dictates the routing capabilities of the switch fabric. Default system licensing is sufficient for pure Layer 2 environments and basic management. However, if deploying a Layer 3 leaf-spine architecture with OSPFv2, EIGRP, BGP, or IS-IS, the Essential License (N3K-ES-XM) is an absolute requirement. Furthermore, if you are integrating telemetry features like Buffer Drop Capture (BDC) or In-band Network Telemetry (INT) for proactive network analytics, you must provision the Essential License.
| Software Package | Features Supported |
|---|---|
| System default (no license required) | Comprehensive Layer 2 (VLAN, LACP, STP, vPC), AAA, ACLs, CoPP, DCNM support, SSHv2, SNMP, Syslog, Advanced buffer monitoring, SPAN, ERSPAN |
| Base license | Layer 3 IP routing: IVR, Static routes, RIPv2, ACLs, OSPFv2 (256 routes), EIGRP stub, HSRP, VRRP |
| Essential License (N3K-ES-XM) | Advanced Layer 3 IP routing: OSPFv2, EIGRP, BGP, IS-IS, VRF-Lite. DCNM and Telemetry: Buffer Drop Capture (BDC), High Delay Capture (HDC), INT |
| Cisco Nexus Data Broker license (NDB-FX-SWT-K9) | Tap and SPAN aggregation functions with Cisco Nexus Data Broker (requires Essential License) |
Power Supplies, Cooling, & Hardware Specs
Power provisioning for the Nexus 3408-S requires strict adherence to redundant (2) power supply planning based on rack power density. The chassis supports 2KW AC, DC, and HVDC power supplies. Select the NXA-PAC-2KW-PI (AC) for standard enterprise data centers, while the NXA-PDC-2KW-PI (DC) or NXA-PHV-2KW-PI (HVDC) is mandatory for telecommunications environments adhering to specific DC power standards. All cooling operates on a forward airflow (port-side intake) design via six individual, hot-swappable fans (5+1 redundant); verify that your hot-aisle/cold-aisle containment strategy aligns with port-side intake to prevent thermal throttling.
| Component / Spec | Details |
|---|---|
| NXA-PAC-2KW-PI | Nexus 2KW AC power supply, forward airflow (port-side intake) |
| NXA-PDC-2KW-PI | Nexus 2KW DC power supply, forward airflow (port-side intake) |
| NXA-PHV-2KW-PI | Nexus 2KW HV power supply, forward airflow (port-side intake) |
| NXA-FAN-300CFM-PI | Nexus fan, forward airflow (port-side intake) |
| Power Supply Requirements | 2 (redundant), 100 to 240 VAC, 50 to 60 Hz, 89 to 91% efficiency at 220V |
| Fans | Six individual, hot-swappable fans (5+1 redundant) |
| Dimensions (H x W x D) | 6.97 in. x 17.3 in. x 31.6 in. |
| Weight (Empty) | 68 lb (without LEMs, PSUs, or fans) |
| Weight (Fully Loaded) | 107.7 lb (with eight 100G LEMs), 104.5 lb (with eight 400G LEMs) |
Technical FAQ
Q: What are the exact breakout capabilities for the 400G Line Expansion Module (NXM-X4D)?
A: The NXM-X4D supports 4 ports of QSFP-DD, which can be broken out into 2x200G, 2x50G, and 4×100/50/25/10G configurations to flexibly connect to existing server access switches or high-density compute nodes.
Q: Which telemetry features become available with the Essential License (N3K-ES-XM)?
A: The Essential License unlocks advanced hardware-level telemetry, specifically enabling Buffer Drop Capture (BDC), High Delay Capture (HDC), and In-band Network Telemetry (INT) for granular network analytics.
Q: What is the dynamic buffer allocation, and how does it assist with microbursts?
A: The Nexus 3408-S features a 70 MB dynamic buffer allocation, allowing the switch to absorb transient traffic microbursts common in distributed storage (RoCEv2) or machine learning environments without dropping packets.
Q: How many power supplies are required, and what is the cooling design?
A: The chassis requires 2 redundant 2KW AC, DC, or HVDC power supplies. Cooling is handled by six hot-swappable fans in a 5+1 redundant configuration utilizing strict forward airflow (port-side intake).