NIC physical layer or data link layer? 2 deferent answer!

yrwinsyrwins Posts: 81Member ■■□□□□□□□□
Hi I'm on Exam Cram book it said that it on physical layer.
but on cbtnuggets it's on data link because it have a MAC address built in.
can some one tell me what it cisco recommendation ?


  • clarsonclarson Posts: 885Member ■■■■□□□□□□
    I'd say a nic supports both layers. So, seeing what the questions are would be helpful in giving an explanation.
    many devices support more than one layer. Such as a router supports the physical, data link, and network layers.
  • anfearranfearr Posts: 27Member ■□□□□□□□□□
    It's both, in contains a MAC address (Layer 2) and transmits the bits (Layer 1).
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  • yrwinsyrwins Posts: 81Member ■■□□□□□□□□
    So for the exam what cisco is recommending.?
  • james43026james43026 Posts: 303Member
    A NIC operates at both layers. And Cisco will most likely agree with that. I'm not sure what Cisco considers correct. But it's a pretty safe bet to assume that they would agree with everyone on this one. As the device has a MAC address hardcoded into it's firmware, which is by it's very definition, a layer 2 concept. Of course one could argue that the NIC operates at layer 1, with the drivers being what actually operates at layer 2, and the drivers operate outside of the NIC within your OS. But the deeper into details you go, when it comes to networking, the more the lines can blur at times. I've never seen anyone successfully prove the argument that a NIC is only a layer 1 device, if that helps at all.
  • yrwinsyrwins Posts: 81Member ■■□□□□□□□□
    thanks James.
    that is very clear to me.
  • LaPantallaLaPantalla Posts: 2Registered Users ■□□□□□□□□□
    It's both layers, it's the lower layer of the data link sub layer ( The mac sub layer ) and the physical layer.
  • EotnakEotnak Posts: 11Member ■□□□□□□□□□
    Technically a NIC is Layer 1 and 2. In an exam, without further detail, when asked, "what layer does a NIC operate at" it would be layer 1. You have to choose the most appropriate answer. If the question were, "what layer does a NIC's MAC address operate at" it would be layer 2.

    Consider that a router operates at ALL layers, but is considered a layer 3 device without further details.
  • james43026james43026 Posts: 303Member
    Eotnak wrote: »
    Technically a NIC is Layer 1 and 2. In an exam, without further detail, when asked, "what layer does a NIC operate at" it would be layer 1. You have to choose the most appropriate answer. If the question were, "what layer does a NIC's MAC address operate at" it would be layer 2.

    Consider that a router operates at ALL layers, but is considered a layer 3 device without further details.

    As you see, there are always multiple schools of thought on what layer a NIC operates at. Consider that many credible sources would say that it operates up to layer 2. I will disagree with your generalization of a router. Not all routers operate above layer 3. If you take a router that literally just routes traffic with no other features enabled or available, it would be operating at layer 3 at best. Now, can a router operate at a layer higher than layer 3, most certainly it can. But saying that they all do, is just wrong. After all routers are intended to only route traffic, and nothing more, it was Cisco that made it popular to add features that other devices traditionally handled, to the router itself. That would be like saying, that because some switches operate up to layer 3, that they are all layer 3 devices. I'm not trying to argue, but just wanted to clarify some things.
  • networker050184networker050184 Posts: 11,962Mod Mod
    Even then it's fairly arguable. If a router is running a routing protocol that communicates via TCP is it just operating at L3? The OSI is just a model. Everything doesn't fit perfectly in it.
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  • james43026james43026 Posts: 303Member
    I'm approaching this from the perspective of what the routers purpose is. And how deep it goes into the inspection of a frame / packet and or what it basis it's routing and forwarding decisions on. Yes, if you look at it from the perspective of what a router is doing outside of routing / forwarding, then we could say that it is operating at layers higher than layer 3, even when all it is doing is routing and forwarding, depending on circumstances, like the one you presented. I also think this is why most literature will always refer to a router as a layer 3 device, because it's from the perspective of what the purpose of the router is, and what exactly it's doing. Just as I would consider a router running some kind of access list or PBR to filter traffic using NBAR, a router that is operating at layer 7. I understand that the OSI model is just theoretical model, and that is a rather large generalization.
  • clarsonclarson Posts: 885Member ■■■■□□□□□□
    Then wouldn't a nic be considered a layer 3 device too.
    if the frame is destined for it's own network, it sends it to the switch. Not on it's network, it sends it to it's default gateway, or a router.
    Pretty simple routing table, to the switch else the router. Still making a decision on where every frame is going. And, it depends on what the network address is. it doesn't depend on the mac address at all. Yes, it looks up the mac address using the ip address, or uses arp (via the ip address) to get the mac address to send the frame to the switch which decides via the mac address which port to forward the frame.
    Then you get into weather routing between devices is equivalent to routing between networks.
  • james43026james43026 Posts: 303Member
    You NIC doesn't care about the IP header of the packet, it only cares about the destination MAC address of the frame. So it would still only be a layer 2 device. Anything above layer 2 would be handled strictly by your devices OS, on Windows that would be WINSOCK and the TCP/IP stack for example.
  • clarsonclarson Posts: 885Member ■■■■□□□□□□
    Same thing applies to a router. All the routing occurs at the OS level and not at the network port.
    A nic needs a computer with an OS to be functional. Yes, you can say certain parts of the OSI model are occurring here or there in the computer. You have all 7 layers on a computer. Yet they will point out one piece of hardware that can be removed from the computer and say this is where layer 2 occurs.
    But, the same is true of a router. The wics usual do the layer two stuff on a router. And, all the level 3 or routing only occurs in the ios or the OS that is running on the router. Yet, they don't say anything about the removable wic being layer 2. and reference the whole router as layer 3, yet they do other layers too.
    They just seem to be using generalizations. routers are layer 3 because routing occurs at that layer. And, switches are layer 2 because that is where switching occurs. And nics are layer 1 because that is where the cable plugs into.
    So, your being asked to pick the best answer to a vague question about generalizations that aren't necessarily true. where is the fairness in that. and how does that measure your knowledge.
  • OctalDumpOctalDump Posts: 1,722Member
    What about a NIC with TCP offload? Or iSCSI HBA?
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  • EotnakEotnak Posts: 11Member ■□□□□□□□□□
    I made my former post from the perspective of a Cisco Net Academy exam. I literally had that question (paraphrased) in an exam and the answer was Layer 1. It was multiple choice, and layer 2 was also an option. Given the choice between the two, I would think most of us would choose layer 1.
  • james43026james43026 Posts: 303Member
    I agree that some routers do make routing decisions in their OS, and thus at layer 7. There are also routers that use ASIC's and TCAM to make routing decisions. Does that mean they are level 2 devices? No! Even if a router is making a decision on how to route a packet in the OS, that doesn't mean that it's routing at any layer higher or lower than layer 3 in either case of software vs hardware based routing. Routing is a layer 3 concept, and will continue to be. As Networker said earlier, not everything fits neatly into the OSI model. It is after all just a concept, a framework, and only meant to be a generalization of sorts. This means that while a router does indeed start by making all decisions in software, and even if those decisions are loaded into hardware and thus routed at a hardware level, or the decisions stay at the software level, routing will stay at layer 3, because the decisions are all based off of layer 3 information. As OctalDump pointed out, a NIC running TCP offload essentially operates at layer 4, this is because the NIC is now diving into each frame / packet / segment up to the layer 4 header, the same applies to a router, it only cares about information going up to layer 3, when it is only routing, and not running any other features. Everyone needs to think about the fact that the OSI model is all about network communication, and nothing else, it was designed to show a framework for how information is passed and processed over a network, and nothing more.
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