If you are building a DGX Spark cluster on a MikroTik CRS804-4DDQ, the cable answer is short: two MCP7H60-W002 breakout DACs put four Sparks on two of the switch's four 400G ports, for $358.90 in cabling, and leave the other two ports free. Each cable plugs its QSFP-DD end into the switch and one QSFP56 leg into each of two Sparks. The one setting that trips people up is on the switch: the CRS804 does not auto-negotiate 2×200G, so you set the speed by hand. Everything you need is below.
Short answer: two breakout cables
Four DGX Sparks on a CRS804 — $358.90 in cabling at Resilient Tec
One 400G QSFP-DD to 2×200G QSFP56 passive breakout DAC feeds two Sparks from one switch port. Two cables cover four nodes; four cables give every Spark both of its ports (dual-rail).
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4-Spark kit: 2 × MCP7H60-W002 — 400G QSFP-DD to 2×200G QSFP56, 2 m Four nodes on two switch ports. Two ports left for growth or storage. |
$358.90 · In stock at Resilient Tec Add 2 to cart |
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MCP7H60-W002 — single cable, 2 m One switch port, two Sparks. |
$179.45 Add to cart |
Resilient Tec sells the MCP7H60-W002 for $179.45 directly at resilient-tec.com — new, lifetime warranty, free standard shipping on all US orders, shipping from Knoxville, Tennessee. Prices checked November 9, 2026. Different switch or node count? Send us your switch model. POs and net terms available.
Why the CRS804 Became the Default DGX Spark Switch
NVIDIA officially supports up to eight DGX Sparks in a switched cluster (announced October 22, 2026). NVIDIA Sync still sets up as many as three cabled directly and as many as four through a switch; larger clusters are configured by hand. (Owners also run four as a switchless ring with custom NCCL software, outside NVIDIA support — the switch is the supported route.) For the fourth node you need something with 200G-capable ports, and the MikroTik CRS804-4DDQ is the cheapest thing that fits: four 400G QSFP56-DD ports and two 10G management ports in a half-width 1U box, at a $1,295 list price. Exxact's four-Spark build guide uses it, and it is the switch most GB10 owners on the NVIDIA forums end up with.
Its limit is port count. Four ports is exactly one per Spark, which leaves nothing for a NAS, an uplink, or a second rail. Breakout cables solve that: one 400G port splits into two 200G legs, which is exactly what a Spark's ConnectX-7 port runs at.
The Port Math: CRS804 and CRS812
The CRS812-8DS-2DQ-2DDQ, MikroTik's other $1,295 400G switch, has only two QSFP56-DD ports plus two 200G QSFP56 ports, so it changes the math. Here is every sensible four-node layout on both switches.
| Switch · layout | Cables | 400G ports used | Ports left | Cabling cost |
|---|---|---|---|---|
| CRS804 · breakout (recommended) | 2 × MCP7H60-W002 | 2 of 4 | 2 × 400G | $358.90 |
| CRS804 · dual-rail (both Spark ports) | 4 × MCP7H60-W002 | 4 of 4 | None | $717.80 |
| CRS804 · straight cables | 4 × MCP1650-V002E26 200G DAC, 2 m | 4 of 4 | None | $460 (4 × $115) |
| CRS812 · breakout | 2 × MCP7H60-W002 | 2 of 2 | 2 × 200G QSFP56 | $358.90 |
On a CRS804 the breakout layout costs about $100 less than straight cables ($358.90 against $460) and keeps half the switch free. Straight Ethernet-coded MCP1650-V cables still make sense if you want each Spark on its own independent cable or the Sparks sit at different distances from the switch — they come in 1 m ($97), 1.5 m ($104), 2 m ($115) and 3 m ($145). On a CRS812 breakout is the only way to land four Sparks on the 400G ports, and the two 200G QSFP56 ports stay open for storage or an uplink. Dual-rail — every Spark using both of its QSFP ports — needs four breakout cables and all four CRS804 ports; operators running it report a little over 100 Gb/s per rail.
Which End Goes Where
The QSFP-DD head goes in the switch. Each QSFP56 leg goes in one Spark port. The Spark's ConnectX-7 cages accept QSFP56 and link at 200G. Never try it the other way round: a Spark port will not split into two links, and operators who have tried report only the first lane coming up. We cover that in Can You Build a 4-Node DGX Spark Cluster Without a Switch?
Length: 2 m (26 AWG) reaches from a desk shelf or a short rack to the switch with slack to spare. The 1 m and 1.5 m (MCP7H60-W01AR30) versions are 30 AWG and more flexible for a stacked shelf; 3 m reaches an adjacent rack. See MCP7H60-W001R30 vs W002R26 vs W003R26 for the full part-number breakdown.
The RouterOS Settings That Make the Legs Link
Plug a breakout DAC into a CRS804 or CRS812 QSFP56-DD port and the switch sees the cable but reports auto-negotiation failed and no link. The fix, documented by Petronella Technology Group's CRS812 build, is to set the speed on the lane-master interfaces of each DD port — lanes 1 and 5:
/interface ethernet
set qsfp56-dd-1-1 auto-negotiation=no speed=200G-baseCR4
set qsfp56-dd-1-5 auto-negotiation=no speed=200G-baseCR4
set qsfp56-dd-2-1 auto-negotiation=no speed=200G-baseCR4
set qsfp56-dd-2-5 auto-negotiation=no speed=200G-baseCR4Then set jumbo frames on the switch as well as the hosts (l2mtu=9216 mtu=9000 on every fabric port) and confirm with ping -M do -s 8972 between every pair of nodes.
Two more settings separate a working link from a fast one. A July 2026 write-up on a CRS804 cluster traced a hard ~10 Gb/s ceiling to unused breakout sub-ports sitting in the bridge, which blocked hardware offload, and to PFC and trust settings stalling RoCE. Its fix removed the QSFP56-DD sub-ports from the management bridge and disabled PFC and L3 trust on them. After tuning, published four-node results on this kind of setup land around 23–24 GB/s NCCL all-reduce bus bandwidth. The full troubleshooting sequence is in DGX Spark Stuck at 100 Gb/s on a Breakout?
Beyond Four Nodes
NVIDIA has supported up to eight DGX Sparks in one cluster since October 22, 2026. NVIDIA Sync still sets up as many as four through a switch; past that you configure by hand. One CRS804 with four MCP7H60 breakouts can feed eight Sparks at 200G (the switch is then full). Spare ports on a four-Spark build are still useful: a 200G link to a NAS, an uplink to the rest of your network, or the second rail for each Spark. The port math is simple — each MCP7H60 breakout adds two Spark ports.
Quick Checklist
- CRS804-4DDQ or CRS812-8DS-2DQ-2DDQ switch
- 2 × MCP7H60-W002 for four Sparks on one rail; 4 for dual-rail on a CRS804 — or 4 × MCP1650-V straight cables on a CRS804 if you want one cable per Spark
- QSFP-DD end in the switch, QSFP56 legs in the Sparks — never split a Spark port
- Auto-negotiation off, speed=200G-baseCR4 on lanes 1 and 5 of each DD port
- MTU 9000 on switch and hosts; test with
ping -M do -s 8972 - Unused breakout sub-ports out of the bridge; PFC off unless you have a reason for it
Update · November 2026
New customer data (Nov 2026): on a 4× GB10 cluster with a MikroTik CRS812, two 400G-to-2×200G breakout cables at the switch matched four individual cables (about 196 Gb/s per link, NCCL all-reduce 24.10 vs 24.12 GB/s) and freed two switch ports. Useful context if you are choosing between one cable per Spark and QSFP-DD breakouts on a CRS804 or CRS812. See part two of our ring vs. switch benchmarks.
FAQ
What cable connects a DGX Spark to a MikroTik CRS804?
A 400G QSFP-DD to 2×200G QSFP56 passive breakout DAC such as the MCP7H60-W002, with the QSFP-DD end in the switch and one QSFP56 leg in each Spark. Two cables connect four Sparks using two of the CRS804's four ports. Resilient Tec sells the MCP7H60-W002 for $179.45, new with a lifetime warranty (price checked November 2, 2026).
How many breakout cables do I need for four DGX Sparks?
Two for a single-rail cluster — each cable serves two Sparks. Four if you want every Spark to use both of its QSFP ports (dual-rail), which fills all four ports on a CRS804.
Why won't my breakout cable link on the CRS804?
The QSFP56-DD ports do not auto-negotiate 2×200G. Turn auto-negotiation off and set speed=200G-baseCR4 on the lane-master interfaces of each DD port (lanes 1 and 5, e.g. qsfp56-dd-1-1 and qsfp56-dd-1-5).
Breakout or straight cables on a CRS804?
Breakout. Two breakout cables ($358.90) cost about $100 less than four straight 2 m MCP1650-V002E26 DACs ($460) and leave two 400G ports free instead of none.
Does the MCP7H60-W002 work on the CRS812?
Yes — on its two QSFP56-DD ports, with the same manual speed setting. That is the only way to put four Sparks on the CRS812's 400G ports, and it leaves the two 200G QSFP56 ports free.
Ready to Buy?
New, lifetime warranty backed by Resilient Tec, free standard shipping on all US orders, shipping from Knoxville, Tennessee. Prices checked November 9, 2026.
- MCP7H60-W002 — 400G QSFP-DD to 2×200G QSFP56 breakout DAC, 2 m — $179.45 each; add the 4-Spark kit (2 cables) or the dual-rail kit (4 cables)
- MCP1650-V — 200G QSFP56 straight DAC, one per Spark — 1 m $97 · 1.5 m $104 · 2 m $115 · 3 m $145
Not on a MikroTik? Send us your switch model and node count and we will confirm exactly what to buy.
You Might Also Need
- Can You Build a 4-Node DGX Spark Cluster Without a Switch? — switch vs. switchless ring at four nodes, and how to cable each
- MCP7H60-W001R30 vs W002R26 vs W003R26 — which length and which part number
- DGX Spark stuck at 100 Gb/s on a breakout? — the tuning that gets you to 200G
- DGX Spark interconnect reference — approved part numbers at two, three and four nodes
- Lossless Ethernet for RoCE — what to get right once a switch is in the path
Sources
- MikroTik CRS804-4DDQ product page
- MikroTik CRS812-8DS-2DQ-2DDQ product page
- Exxact — Build a 4x NVIDIA DGX Spark Cluster
- Petronella Technology Group — DGX Spark Cluster: From Two Sparks to a Switched Fabric
- IT Admin Blog — DGX Spark RoCE CRS804 fix
- NVIDIA Developer Forums — ConnectX-7 200GbE via MikroTik CRS812 and QSFP-DD breakout
- NVIDIA MCP7H60-W0xxRyy specifications