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> The only reason googling shows so much confusion is most people don't understand the definitions of routing and switching. No, it means that people are using

> The only reason googling shows so much confusion is most people don't understand the definitions of routing and switching. No, it means that people are using a broader definition that is not locked into the OSI model. Both definitions are valid but the OSI one is a subset of the other one. Than means the OSI one can't describe certain network devices that exist in the real world. A layer 3 switch can't exist according to the OSI model. It can in the real world. Go way back to the parent of this thread. See the sentence "Anyway, OSI model is mostly a mental picture that bear little practical relevance."? Yet you're here arguing we have to use the OSI model's definition of switching and routing. > "Layer 2" = switching. "Layer 3" = routing. Only if you think that the OSI model is the be all end all definition of what a network is. It's not. The TCP/IP designers didn't think so either and moved functionality that was traditionally in the lower layers to the upper layers. > "Layer 3 switch" = a marketing term Yes, it's been turned into a marketing term. But to understand why it's a marketing term you need to know what a real layer 3 switch is. Then it becomes readily apparent why they really created is a router with a handful of switching features rolled in for speed. > Your "pure layer 3 switch" that only services a single subnet is actually a regular old layer 2 switch. No it's not. If you could modify the end nodes in the network (you can with Linux or the *BSDs) to use the broadcast MAC address for the destination and have them ignore the ethertype field you'd completely bypass Layer 2. The layer 3 switches would still be able to function and switch packets to the proper nodes. A layer 2 switch in this situation would still work but would broadcast all packets to all nodes and thus stop acting as a switch. > It doesn't need to be aware of "layer 3" at all. If it's not aware of layer 3 at all then it's not a layer 3 switch. A layer 3 switch by definition needs to understand layer 3. So if you had one that just switches using IP (v4 and/or v6) it would not be able to switch other protocols like IPX for example. A layer 2 switch would switch anything.

icedchai 9y ago
Please point me to a definition of a "real layer 3 switch." I would like to know. I've been working with TCP/IP for almost 25 years. It sounds like you want to eliminate the broadcast domain... If you want point-to-point ethernet, put your end nodes on a IPv4 /31 (or /30, depending) and be done with it. You will have a "pure layer 3" environment: all traffic will be routed. No end node modifications required.
- Avernar 9y ago
  > Please point me to a definition of a "real layer 3 switch." All definitions of layer 3 switch I've found were stuff like this: https://en.wikipedia.org/wiki/LAN_switching#Layer_3_switching https://en.wikipedia.org/wiki/LAN_switching#Layer_3_switchin... Those definitions are parroting the manufacturer's definitions. Those devices are routers with ASIC switching using the IP address. My definition of a real layer 3 switch is this: A device that forwards packets based on the IP address. The decision on which port to forward is based on where an IP was last seen and not on a configured network/netmask value. Nobody that I know of sells devices like this but you can build one for yourself. The reason I'm going on about a pure layer 3 switch is because it's on one end of the discussion about these hybrid "Layer 3 Switches" that the network manufacturers are selling. It's like trying to talk about what a mule is when you refuse to define what a donkey is. So we have a pure layer 3 switch on one end and a layer 3 router on the other. The "Layer 3 Switches" that are now being sold are about 95% of the way towards the router side of that line. > If you want point-to-point ethernet, put your end nodes on a IPv4 /31 (or /30, depending) and be done with it. You will have a "pure layer 3" environment: all traffic will be routed. But then you're routing and have different subnets everywhere. You won't be able to transparently replace a layer 2 switch with that. Ethernet framing is redundant. The LLC/SNAP header has already been deprecated by the IEEE so there's 8 bytes saved. The ethertype field is redundant as the first byte of the payload will tell you if it's IP v4 or v6 so there's an extra 2 bytes. The source and dest MAC addresses are redundant as we have source and dest IP addresses so there's a 12 byte savings. The vlan tag would need to stay for now because of IPv4 but we could replace it by repurposing some of the 0 bits in an IPv6 link local address. The preamble, SFD and FCS would stay although the preamble might be able to be shortened as it's minimum length was dictated partly by how many hubs a packet could cross. So you replace your layer 2 switch with this layer 3 switch. Everything still works with no configuration necessary. As new network cards that can do IP framing come out you configure each port from ethernet framing to IP framing. So now it's a Layer 3 switch on some ports (Ehternet framing) and a Layer 2 switch on others (IP Framing). Once all ports are switched over the device has now become a Layer 2 IP switch. Now you can see why I don't like the definition of Layer 2 = Switching, Layer 3 = Routing. The device functions as a switch regardless of the framing on each of the ports. If you use the more general definition of Switching = inside the same network (i.e. subnet) and Routing = between networks then things become more clear. I know some people on the internet want IP framing to become a thing. Will it? Maybe, maybe not. But by using the more rigid OSI definition of switching and routing it becomes harder to talk about topics like that. That's why we have TCP/IP now. Because some people were willing to think outside of the OSI box. > It sounds like you want to eliminate the broadcast domain It wouldn't eliminate it. The broadcasts would eventually become either IPv4 broadcasts or IPv6 multicasts. It would reduce the sizes of the domains and reduce traffic however.

icedchai 9y ago

Please point me to a definition of a "real layer 3 switch." I would like to know. I've been working with TCP/IP for almost 25 years. It sounds like you want to eliminate the broadcast domain... If you want point-to-point ethernet, put your end nodes on a IPv4 /31 (or /30, depending) and be done with it. You will have a "pure layer 3" environment: all traffic will be routed. No end node modifications required.

Avernar 9y ago
> Please point me to a definition of a "real layer 3 switch." All definitions of layer 3 switch I've found were stuff like this: https://en.wikipedia.org/wiki/LAN_switching#Layer_3_switching https://en.wikipedia.org/wiki/LAN_switching#Layer_3_switchin... Those definitions are parroting the manufacturer's definitions. Those devices are routers with ASIC switching using the IP address. My definition of a real layer 3 switch is this: A device that forwards packets based on the IP address. The decision on which port to forward is based on where an IP was last seen and not on a configured network/netmask value. Nobody that I know of sells devices like this but you can build one for yourself. The reason I'm going on about a pure layer 3 switch is because it's on one end of the discussion about these hybrid "Layer 3 Switches" that the network manufacturers are selling. It's like trying to talk about what a mule is when you refuse to define what a donkey is. So we have a pure layer 3 switch on one end and a layer 3 router on the other. The "Layer 3 Switches" that are now being sold are about 95% of the way towards the router side of that line. > If you want point-to-point ethernet, put your end nodes on a IPv4 /31 (or /30, depending) and be done with it. You will have a "pure layer 3" environment: all traffic will be routed. But then you're routing and have different subnets everywhere. You won't be able to transparently replace a layer 2 switch with that. Ethernet framing is redundant. The LLC/SNAP header has already been deprecated by the IEEE so there's 8 bytes saved. The ethertype field is redundant as the first byte of the payload will tell you if it's IP v4 or v6 so there's an extra 2 bytes. The source and dest MAC addresses are redundant as we have source and dest IP addresses so there's a 12 byte savings. The vlan tag would need to stay for now because of IPv4 but we could replace it by repurposing some of the 0 bits in an IPv6 link local address. The preamble, SFD and FCS would stay although the preamble might be able to be shortened as it's minimum length was dictated partly by how many hubs a packet could cross. So you replace your layer 2 switch with this layer 3 switch. Everything still works with no configuration necessary. As new network cards that can do IP framing come out you configure each port from ethernet framing to IP framing. So now it's a Layer 3 switch on some ports (Ehternet framing) and a Layer 2 switch on others (IP Framing). Once all ports are switched over the device has now become a Layer 2 IP switch. Now you can see why I don't like the definition of Layer 2 = Switching, Layer 3 = Routing. The device functions as a switch regardless of the framing on each of the ports. If you use the more general definition of Switching = inside the same network (i.e. subnet) and Routing = between networks then things become more clear. I know some people on the internet want IP framing to become a thing. Will it? Maybe, maybe not. But by using the more rigid OSI definition of switching and routing it becomes harder to talk about topics like that. That's why we have TCP/IP now. Because some people were willing to think outside of the OSI box. > It sounds like you want to eliminate the broadcast domain It wouldn't eliminate it. The broadcasts would eventually become either IPv4 broadcasts or IPv6 multicasts. It would reduce the sizes of the domains and reduce traffic however.