8 ms·

What Is a Direct Attach Copper (DAC) Cable? (2021)

118 points by teleforce 5mo ago

happyPersonR 5mo ago
Dacs are great … they let you avoid having to convert to optical or having to splice your own cable. Downside the cables are kind of heavy and sometimes folks dont like that.
- bombcar 5mo ago
  Main advantage for me was saving the cost of the SPF (surprisingly pricey f^#%rs) on each end.
  - namibj 5mo ago
    Huh, what were you doing, buying Cisco/HPE, or trying to use fancy single mode optics for distances that DACs could do?
    - bombcar 5mo ago
      I just wanted to use my CRS305-1G-4S+IN without spending more on SPF modules than I did on the cards/switch.
      - Henchman21 5mo ago
        SFP* sorry :)
      - simoncion 5mo ago
        Have you looked at what 10gtek is selling? I tried two low-price SFP+ SR, multi-mode module vendors way back when I first got my CRS326-24G-2S+ switches and 10gtek's modules were the one that worked. Newegg is currently selling a four-pack of them for ~52 USD, and singles for ~18 USD. The ones I have run warm (~50C in ~25C ambient) but I've had mine for something like four, maybe five years now and they're still working fine.
Reubachi 5mo ago
While working for a broadcasting/networking integration gig, we designed, assembled and field-terminated thousands of copper and fibre runs from 1/2 ft to football field lengths. The amount of time required to terminate a copper cable in the field is seconds, and felt a bit like art. Something about the way it reliably reacted was magical and felt "strong." Terminating or splicing a fibre cable felt like wrestling a snake covered in melted crayons, and the failure rate was significantly higher across the board. And it wasn't just workmanship, but quality of product, terminating environment, available equipment, misuse by future operators etc. That said, at a certain point, we as a firm learned that most purchasers would rather the low latency/small footprint of optical/fibre versus copper, maintenece/failure be damned. Though, maybe part of our willingness to push fibre came from knowing that most purchasers would in 1-2 years call us back in to replace the rack terminations with copper :)
- zamadatix 5mo ago
  There are 2 very different main types of "copper" in this context. Each can break down into more specific subcategories but they have a relatively common general behavior with their primary type. First there is the BASE-T RJ45 stuff, which it sounds like you might have been working with. At 10G or higher speeds this get relatively power hungry and is not really an advantage over fiber unless you are also delivering PoE or are trying to reuse existing cabling. This type (DAC) is a special type of pre-made cable assembly which eschews much of the advanced signalling/conversion logic. The upside is the power usage is low (often even lower than fiber) and the cost is dirt cheap. The downside is the lengths are much more limited and it's intended to be preterminated SFP-to-SFP (or the like) cable assemblies instead of modular patching/custom built. There is indeed a latency difference, usually DAC < fiber < BASE-T, but they are all within a few microseconds (not milliseconds) of each other so you really have to be pressed to care about it (to the point you're looking at specialized low latency switches and paying extra to lay things out in a way which minimizes the number of L2 hops rather than the cost).
  - myself248 5mo ago
    > it's intended to be preterminated SFP-to-SFP I wonder why nobody's making field-terminatable DACs for custom lengths. If you've torn one down, they're not exactly complicated inside. If they existed, would people buy them?
protocolture 5mo ago
Dacs are awesome until you encounter a full rack with nothing but 2 and 4 meter dacs. They appeal very much to the lazy engineer, and a lazy engineer can turn them into a rats nest.
- kjs3 5mo ago
  To be fair, the dedicated lazy engineer can make a rats nest out of any interconnect technology.
- mercurialuser 5mo ago
  And they can block heat exhaustion
- HappMacDonald 5mo ago
  You.. but you put a ladder on the side. Rat's nest goes in the ladder, cover goes on top of the ladder. That way cables aren't blocking access to add/remove any U's. You make sure all of the cabling has ends that can be pulled safely through the rat nests in the ladders (EG especially RJ45 cabling needs competent clip guards) and Bob's your uncle
  - protocolture 5mo ago
    Thick DAC cables, especially long ones, can be very resistant to cable management. Then theres the deal with trying to detangle it in the first place.
alexfoo 5mo ago
I'm waiting for 3 DACs and a few other bits to arrive today to move closer to 10G networking at home. Moving house soon and the new place will have 2.5Gbps FTTP (both up and down) so I wanted to be prepared for that. Given my existing broadband is only 500/75Mbps FTTP I was fine with a 1GbE internal network and Wifi-6 meshing. I could have planned to move to 2.5GbE but it may have been a bottleneck at some point, so may as well push straight on to 10G. I have a USW-Aggregation with 8 SFP+ ports arriving today too. Just have to install Intel X520-DA2 cards in two of my servers (Proxmox host and a general Linux server), and the NAS also has a 10G SFP+ port, and then connect it all up. Most of it second hand from eBay for half the usual retail price.
- lostlogin 5mo ago
  Nice work, that agg switch is excelllwnt. I went with some cheap eBay cards and slotted them into a synology and PC. They work great and have for years. https://www.ebay.com/itm/384094168784?_skw=connectx+mellanox&epid=28045097649&itmmeta=01KSMNMGD3GCC83J73CDEAHVW1&hash=item596dcbe6d0%3Ag%3ArXkAAeSwCy1qFNpX&itmprp=enc%3AAQALAAAAwGfYFPkwiKCW4ZNSs2u11xBPA1Lc7rxyOZVyJlKU35Y1aMLUGeXc5pSZy0DwQKIwRe0r7D4Ds8EeGbTv%2Fv4sh1%2FlAZ8oFqIKFPzZztuXCw7yP%2BPrcYOR7kZx9zoC%2BsYYpnFxy9ZygAur3a6B%2BnkcRdTjrPaj9IMJL5LRoZKBY4lpEY9gsWNeCrPO%2BDwTqdspx2Bonp9rwjC214QKyjtyhVVVD8tOyQ3n2URke5CnxjAQlUCb9hJgF9tfdTr1WE27aA%3D%3D%7Ctkp%3ABlBMUNqG0pXNZw&keyword=connectx+mellanox&sacat=0&relatedSearch=true https://www.ebay.com/itm/384094168784?_skw=connectx+mellanox...
- Filligree 5mo ago
  How many PCIe lanes are you allocating? The card is obviously 16-lane, but it also has two ports; 40Gb total. In a server that’s fine, but if you want 10G in a desktop you’ll have a problem. I’m probably not telling you anything new. NICs using newer PCI generations are rare as hen’s teeth. It should be possible to do this with four lanes, but isn’t… Unless you find a 25G dual-port card, in which case the single lane my secondary slots hand out does at least suffice for 10G one way.
  - alexfoo 5mo ago
    I'm only planning on using one of the SFP+ ports on each of the cards, the dual port cards were just more common and cheaper on eBay. The specs say they require PCIe v2.1 x8 lane. My Proxmox server is quite old and has a Gigabyte GA-X79-UP4 mobo and has loads of spare PCI slots. One slot is taken up by a generic graphics card as the Mobo has no on-board graphics. (I think I went for this mobo because of the number of SATA ports, but it was over 10 years ago so not entirely sure.) My general Linux server is newer and has an ASUS Prime H610M-A D4 mobo. Only two PCI slots (not used at the moment) and so the Intel X540-DA2 will use up the PCIe 4.0 x16 slot leaving just a PCIe 3.0 x1 slot. But that's fine as this machine is just a CPU (i7-13700), 64GB RAM and a 2TB NVMe. Sticking a good graphics card in it for GPU related fun had been on my list for years but I never got around to it, now the prices are just insane so I'll ignore that for now or something second hand falls into my lap.
throw0101c 5mo ago
> This is a bit dependent on the type of DAC, and the vendor. Generally speaking, we are moving into an era as we move above 100GbE speeds where DACs will be limited to 5m runs and shorter runs. This is generally good enough for intra-rack runs, especially if you put your top-of-rack (ToR) switch in the middle: you basically only have to go about 1m/3' in either direction (up/down).
Shank 5mo ago
I’m a huge fan of fiber optic cables instead of DAC. For one thing, the distance you can run is far greater. For another, they don’t get as hot as metal cables, particularly at high speeds. OM3 and OM4 is actually quite affordable and SFP+ is nice. Yes on an absolute sense, DAC can be cheaper, but for a homelab or something, it’s not that much different. Also, it’s much cooler to run fiber optic to things (imho).
- GoofGarage 5mo ago
  In a homelab scenario when asking “DAC vs. fiber” the answer is usually, “yes.” :) Basically, it’s a tradeoff as with everything else. DACs will usually be even (slightly) lower power per port, and slightly lower latency[1] (we’re fighting over microseconds here!), with excellent durability. The tradeoff is for passive DACs you’re limited on range, cost is often higher, and they may need to be encoded for your interfaces. Moreover, the range is very limited. Fiber (the cable) is immune to electrical noise, can run long distances, advances in wave division multiplexing extends the life of the fiber by changing what’s the fiber connects to. The downside is you pay slightly in latency for media interface changes (the electrical-to-optical conversions), the limits of bend radius of the cable to not break the cable or reduce bandwidth, and the relative complexity of field terminations compared to twisted pair. I’ve 25+ years experience with fiber, and trust me, it’s great. ———— Outside of cost, both crush twisted pair like an ant. The power consumption per port is also far lower. However, this is only going to matter if you focus on limiting power consumption (not for cost, on principle), have very high-bandwidth applications where latency matters (I do!), and/or just want field experience with things other than twisted pair. I use DAC and fiber for some things as I try to get every scrap of capability out of my hardware. For example, I have VERY low power (silent or near silent) hardware where I can push 5GB (so ~40Gbps) / sec storage. Not just sending it over the wire, but actually committing it to disk without buffering in RAM. So I have the capability of “PCIe 3.0/4.0 x4 NVMe” speeds across the network… from the (mostly silent) storage server, to anything else that can send or ingest the data that fast. Despite the storage server having very little flash (a few TB vs 100TB+ disk). That’s harder to do with twisted pair, or at least the power consumption of the network connectivity itself starts to add up for a few virtualization cluster nodes. ———- Generally, “DAC in the rack, fiber to out back” is a reasonable approach. Though “fiber-only” works if you want to limit complexity! [1] Fiber and DAC tend to trade places on latency every generation or so. It’s a very close race, but they crush twisted pair.
kohlschuetter 5mo ago
Many 10GbE DACs can be hacked to run 25GbE, which is something I had to figure out after realizing that it’s not just plug and play as it was with twisted pair cables coming from 1Gb to 2.5Gb… https://kohlschuetter.github.io/blog/posts/2026/03/22/unlock25/ https://kohlschuetter.github.io/blog/posts/2026/03/22/unlock...
- ragall 5mo ago
  Thanks! That's very useful, I might have to do the same soon.
Havoc 5mo ago
Surprised it doesn't mention heat - one of the big advantages. There is also a new generation of "invisible" fiber that is interesting for home use. Basically looks like fishing line so you can run it somewhere that you just don't see unless you're really looking. Fragile tho
jeffrallen 5mo ago
Too bad they didn't mention that while the cable is direct attach, the ends each have an EEPROM in them saying what the cable is. This EEPROM can be a source of immense frustration, if it's got the wrong data in it, or if both ends do not agree.
- 83 5mo ago
  This should be higher up for anyone considering DACs for homelab use. I had a lot of issues due to this when mixing Aruba switches with Intel nics in my rack. Ended up going to fiber and SFPs.
anonymousiam 5mo ago
I picked up one of these just a few days ago for use with some SFP28 ports. They're cheap, fast, and don't use any power.
amelius 5mo ago
Is this connection galvanically isolated?
- userbinator 5mo ago
  No.
  - amelius 5mo ago
    OK smells like problems then.
why_at 5mo ago
Worth mentioning that this is from 2021 and there are now faster speeds and more options. We have OSFP/QSFPDD with 8 lanes and lane speeds of 100Gpbs and 200Gbps for up to 1.6Tbps on a single DAC at the bleeding edge. Although with these signal integrity limits the maximum length to about 1m. You probably won't see this on a NIC or anything for home use any time soon though.

118 points by teleforce 5mo ago

Dacs are great … they let you avoid having to convert to optical or having to splice your own cable. Downside the cables are kind of heavy and sometimes folks dont like that.

bombcar 5mo ago
Main advantage for me was saving the cost of the SPF (surprisingly pricey f^#%rs) on each end.
- namibj 5mo ago
  Huh, what were you doing, buying Cisco/HPE, or trying to use fancy single mode optics for distances that DACs could do?
  - bombcar 5mo ago
    I just wanted to use my CRS305-1G-4S+IN without spending more on SPF modules than I did on the cards/switch.
    - Henchman21 5mo ago
      SFP* sorry :)
    - simoncion 5mo ago
      Have you looked at what 10gtek is selling? I tried two low-price SFP+ SR, multi-mode module vendors way back when I first got my CRS326-24G-2S+ switches and 10gtek's modules were the one that worked. Newegg is currently selling a four-pack of them for ~52 USD, and singles for ~18 USD. The ones I have run warm (~50C in ~25C ambient) but I've had mine for something like four, maybe five years now and they're still working fine.

Reubachi 5mo ago

While working for a broadcasting/networking integration gig, we designed, assembled and field-terminated thousands of copper and fibre runs from 1/2 ft to football field lengths. The amount of time required to terminate a copper cable in the field is seconds, and felt a bit like art. Something about the way it reliably reacted was magical and felt "strong." Terminating or splicing a fibre cable felt like wrestling a snake covered in melted crayons, and the failure rate was significantly higher across the board. And it wasn't just workmanship, but quality of product, terminating environment, available equipment, misuse by future operators etc. That said, at a certain point, we as a firm learned that most purchasers would rather the low latency/small footprint of optical/fibre versus copper, maintenece/failure be damned. Though, maybe part of our willingness to push fibre came from knowing that most purchasers would in 1-2 years call us back in to replace the rack terminations with copper :)

zamadatix 5mo ago
There are 2 very different main types of "copper" in this context. Each can break down into more specific subcategories but they have a relatively common general behavior with their primary type. First there is the BASE-T RJ45 stuff, which it sounds like you might have been working with. At 10G or higher speeds this get relatively power hungry and is not really an advantage over fiber unless you are also delivering PoE or are trying to reuse existing cabling. This type (DAC) is a special type of pre-made cable assembly which eschews much of the advanced signalling/conversion logic. The upside is the power usage is low (often even lower than fiber) and the cost is dirt cheap. The downside is the lengths are much more limited and it's intended to be preterminated SFP-to-SFP (or the like) cable assemblies instead of modular patching/custom built. There is indeed a latency difference, usually DAC < fiber < BASE-T, but they are all within a few microseconds (not milliseconds) of each other so you really have to be pressed to care about it (to the point you're looking at specialized low latency switches and paying extra to lay things out in a way which minimizes the number of L2 hops rather than the cost).
- myself248 5mo ago
  > it's intended to be preterminated SFP-to-SFP I wonder why nobody's making field-terminatable DACs for custom lengths. If you've torn one down, they're not exactly complicated inside. If they existed, would people buy them?

protocolture 5mo ago

Dacs are awesome until you encounter a full rack with nothing but 2 and 4 meter dacs. They appeal very much to the lazy engineer, and a lazy engineer can turn them into a rats nest.

kjs3 5mo ago
To be fair, the dedicated lazy engineer can make a rats nest out of any interconnect technology.
mercurialuser 5mo ago
And they can block heat exhaustion
HappMacDonald 5mo ago
You.. but you put a ladder on the side. Rat's nest goes in the ladder, cover goes on top of the ladder. That way cables aren't blocking access to add/remove any U's. You make sure all of the cabling has ends that can be pulled safely through the rat nests in the ladders (EG especially RJ45 cabling needs competent clip guards) and Bob's your uncle
- protocolture 5mo ago
  Thick DAC cables, especially long ones, can be very resistant to cable management. Then theres the deal with trying to detangle it in the first place.

alexfoo 5mo ago

I'm waiting for 3 DACs and a few other bits to arrive today to move closer to 10G networking at home. Moving house soon and the new place will have 2.5Gbps FTTP (both up and down) so I wanted to be prepared for that. Given my existing broadband is only 500/75Mbps FTTP I was fine with a 1GbE internal network and Wifi-6 meshing. I could have planned to move to 2.5GbE but it may have been a bottleneck at some point, so may as well push straight on to 10G. I have a USW-Aggregation with 8 SFP+ ports arriving today too. Just have to install Intel X520-DA2 cards in two of my servers (Proxmox host and a general Linux server), and the NAS also has a 10G SFP+ port, and then connect it all up. Most of it second hand from eBay for half the usual retail price.

lostlogin 5mo ago
Nice work, that agg switch is excelllwnt. I went with some cheap eBay cards and slotted them into a synology and PC. They work great and have for years. https://www.ebay.com/itm/384094168784?_skw=connectx+mellanox&epid=28045097649&itmmeta=01KSMNMGD3GCC83J73CDEAHVW1&hash=item596dcbe6d0%3Ag%3ArXkAAeSwCy1qFNpX&itmprp=enc%3AAQALAAAAwGfYFPkwiKCW4ZNSs2u11xBPA1Lc7rxyOZVyJlKU35Y1aMLUGeXc5pSZy0DwQKIwRe0r7D4Ds8EeGbTv%2Fv4sh1%2FlAZ8oFqIKFPzZztuXCw7yP%2BPrcYOR7kZx9zoC%2BsYYpnFxy9ZygAur3a6B%2BnkcRdTjrPaj9IMJL5LRoZKBY4lpEY9gsWNeCrPO%2BDwTqdspx2Bonp9rwjC214QKyjtyhVVVD8tOyQ3n2URke5CnxjAQlUCb9hJgF9tfdTr1WE27aA%3D%3D%7Ctkp%3ABlBMUNqG0pXNZw&keyword=connectx+mellanox&sacat=0&relatedSearch=true https://www.ebay.com/itm/384094168784?_skw=connectx+mellanox...
Filligree 5mo ago
How many PCIe lanes are you allocating? The card is obviously 16-lane, but it also has two ports; 40Gb total. In a server that’s fine, but if you want 10G in a desktop you’ll have a problem. I’m probably not telling you anything new. NICs using newer PCI generations are rare as hen’s teeth. It should be possible to do this with four lanes, but isn’t… Unless you find a 25G dual-port card, in which case the single lane my secondary slots hand out does at least suffice for 10G one way.
- alexfoo 5mo ago
  I'm only planning on using one of the SFP+ ports on each of the cards, the dual port cards were just more common and cheaper on eBay. The specs say they require PCIe v2.1 x8 lane. My Proxmox server is quite old and has a Gigabyte GA-X79-UP4 mobo and has loads of spare PCI slots. One slot is taken up by a generic graphics card as the Mobo has no on-board graphics. (I think I went for this mobo because of the number of SATA ports, but it was over 10 years ago so not entirely sure.) My general Linux server is newer and has an ASUS Prime H610M-A D4 mobo. Only two PCI slots (not used at the moment) and so the Intel X540-DA2 will use up the PCIe 4.0 x16 slot leaving just a PCIe 3.0 x1 slot. But that's fine as this machine is just a CPU (i7-13700), 64GB RAM and a 2TB NVMe. Sticking a good graphics card in it for GPU related fun had been on my list for years but I never got around to it, now the prices are just insane so I'll ignore that for now or something second hand falls into my lap.

throw0101c 5mo ago

> This is a bit dependent on the type of DAC, and the vendor. Generally speaking, we are moving into an era as we move above 100GbE speeds where DACs will be limited to 5m runs and shorter runs. This is generally good enough for intra-rack runs, especially if you put your top-of-rack (ToR) switch in the middle: you basically only have to go about 1m/3' in either direction (up/down).

Shank 5mo ago

I’m a huge fan of fiber optic cables instead of DAC. For one thing, the distance you can run is far greater. For another, they don’t get as hot as metal cables, particularly at high speeds. OM3 and OM4 is actually quite affordable and SFP+ is nice. Yes on an absolute sense, DAC can be cheaper, but for a homelab or something, it’s not that much different. Also, it’s much cooler to run fiber optic to things (imho).

GoofGarage 5mo ago
In a homelab scenario when asking “DAC vs. fiber” the answer is usually, “yes.” :) Basically, it’s a tradeoff as with everything else. DACs will usually be even (slightly) lower power per port, and slightly lower latency[1] (we’re fighting over microseconds here!), with excellent durability. The tradeoff is for passive DACs you’re limited on range, cost is often higher, and they may need to be encoded for your interfaces. Moreover, the range is very limited. Fiber (the cable) is immune to electrical noise, can run long distances, advances in wave division multiplexing extends the life of the fiber by changing what’s the fiber connects to. The downside is you pay slightly in latency for media interface changes (the electrical-to-optical conversions), the limits of bend radius of the cable to not break the cable or reduce bandwidth, and the relative complexity of field terminations compared to twisted pair. I’ve 25+ years experience with fiber, and trust me, it’s great. ———— Outside of cost, both crush twisted pair like an ant. The power consumption per port is also far lower. However, this is only going to matter if you focus on limiting power consumption (not for cost, on principle), have very high-bandwidth applications where latency matters (I do!), and/or just want field experience with things other than twisted pair. I use DAC and fiber for some things as I try to get every scrap of capability out of my hardware. For example, I have VERY low power (silent or near silent) hardware where I can push 5GB (so ~40Gbps) / sec storage. Not just sending it over the wire, but actually committing it to disk without buffering in RAM. So I have the capability of “PCIe 3.0/4.0 x4 NVMe” speeds across the network… from the (mostly silent) storage server, to anything else that can send or ingest the data that fast. Despite the storage server having very little flash (a few TB vs 100TB+ disk). That’s harder to do with twisted pair, or at least the power consumption of the network connectivity itself starts to add up for a few virtualization cluster nodes. ———- Generally, “DAC in the rack, fiber to out back” is a reasonable approach. Though “fiber-only” works if you want to limit complexity! [1] Fiber and DAC tend to trade places on latency every generation or so. It’s a very close race, but they crush twisted pair.

kohlschuetter 5mo ago

Many 10GbE DACs can be hacked to run 25GbE, which is something I had to figure out after realizing that it’s not just plug and play as it was with twisted pair cables coming from 1Gb to 2.5Gb… https://kohlschuetter.github.io/blog/posts/2026/03/22/unlock25/ https://kohlschuetter.github.io/blog/posts/2026/03/22/unlock...

ragall 5mo ago
Thanks! That's very useful, I might have to do the same soon.

Havoc 5mo ago

Surprised it doesn't mention heat - one of the big advantages. There is also a new generation of "invisible" fiber that is interesting for home use. Basically looks like fishing line so you can run it somewhere that you just don't see unless you're really looking. Fragile tho

jeffrallen 5mo ago

Too bad they didn't mention that while the cable is direct attach, the ends each have an EEPROM in them saying what the cable is. This EEPROM can be a source of immense frustration, if it's got the wrong data in it, or if both ends do not agree.

83 5mo ago
This should be higher up for anyone considering DACs for homelab use. I had a lot of issues due to this when mixing Aruba switches with Intel nics in my rack. Ended up going to fiber and SFPs.

anonymousiam 5mo ago

I picked up one of these just a few days ago for use with some SFP28 ports. They're cheap, fast, and don't use any power.

amelius 5mo ago

Is this connection galvanically isolated?

userbinator 5mo ago
No.
- amelius 5mo ago
  OK smells like problems then.

why_at 5mo ago

Worth mentioning that this is from 2021 and there are now faster speeds and more options. We have OSFP/QSFPDD with 8 lanes and lane speeds of 100Gpbs and 200Gbps for up to 1.6Tbps on a single DAC at the bleeding edge. Although with these signal integrity limits the maximum length to about 1m. You probably won't see this on a NIC or anything for home use any time soon though.