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I'm exploring alternate multiplicities between IP subnets and VLANs, outside the recommended 1-to-1 implementation. My understanding is as follows:

Multiple subnets to a single VLAN (connected via a switch):

  • Hosts across both subnets would receive layer 2 broadcasts (such as ARP), but would ignore traffic lacking an IP that targets them.

Question: Would I be able to communicate across subnets without a layer 3 device if I could manually insert a destination MAC address in the frame header? My understanding is that the layer 2 switch is oblivious to the differing subnets, and assuming it knows the location of the destination MAC address, would forward the packet in its direction. The destination PC, seeing its IP and MAC addresses, would accept the packet, effectively letting it cross subnets without ever being routed.

A single subnet across multiple VLANs:

  • Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same subnet (but unknowingly in another VLAN) would send out an ARP request that would never be responded to.
    This would effectively create separate, identical address pools for each of the VLANs, though I'm not sure how a router would differentiate between the two when interVLAN communication is attempted. I'm a little bit unsure about the pros/cons of this configuration..

Why would we ever want to do this?

Community
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powrHouse
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2 Answers2

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Multiple subnets to a single VLAN (connected via a switch):

Hosts across both subnets would receive layer 2 broadcasts (such as ARP), > but would ignore traffic lacking an IP that targets them.

This actually has it's use case in modern DCs. Not in a way you suggest it (w/o a L3 device), but with a VEPA switch.

A single subnet across multiple VLANs:

Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same subnet (but unknowingly in another VLAN) would send out an ARP request that would never be responded to. This would effectively create separate, identical address pools for each of the VLANs, though I'm not sure how a router would differentiate between the two when interVLAN communication is attempted. I'm a little bit unsure about the pros/cons of this configuration..

A single subnet across multiple VLANs, also called Transparent subnet gatewaying (RFC 1027) is a somehow archaic approach. It uses Proxy ARP, but proxy ARP has it's own set of problems.

NetworkMeister
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Multiple subnets to a single VLAN (connected via a switch):

  • Hosts across both subnets would receive layer 2 broadcasts (such as ARP), but would ignore traffic lacking an IP that targets them.

Question: Would I be able to communicate across subnets without a layer 3 device if I could manually insert a destination MAC address in the frame header?

You will need to replace the MAC address, and need to recalculate FCS over the whole frame, else the switch will reject it as a damaged frame. This must happen after your ethernet driver does this.

A single subnet across multiple VLANs:

  • Broadcast traffic would be isolated to the individual VLANs. This would break ARP, as a host targeting another machine in the same subnet (but unknowingly in another VLAN) would send out an ARP request that would never be responded to. This would effectively create separate, identical address pools for each of the VLANs, though I'm not sure how a router would differentiate between the two when interVLAN communication is attempted. I'm a little bit unsure about the pros/cons of this configuration..

Why would we ever want to do this?

Hosts in the same subnet would not be able to communicate with each other. Most routers will not let you assign the same network to multiple interfaces, unless they are bridged interfaces, in which case, you haven't accomplished anything except sending the traffic the long way around.

Some switches have something similar to this, called Private VLANs, where hosts can only communicate with a gateway. This is a security feature used in some situations.

Ron Maupin
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  • Thanks for the response. So regarding the first point, inter-subnet traffic without a router would be possible in this case? – powrHouse May 22 '16 at 05:05
  • Theoretically, but not practically. You would need to grab the traffic as the driver was putting it in the hardware. – Ron Maupin May 22 '16 at 12:10