1

We expect that there is a switch, configured with three VLANs on one port (Trunk).

VLAN10 -> 10.0.1.0/24 
VLAN20 -> 10.0.2.0/24 
VLAN30 -> 10.0.3.0/24

A unmanaged switch is connected to this port, on this switch, there are three devices connected, which all have 3 different IPs.

Assume:

PC A -> 10.0.1.100
PC B -> 10.0.2.100
PC C -> 10.0.3.100

Questions:

  1. Are they on three different VLANs, or is the VLAN tag dropped because it is a unmanaged switch?
  2. What happens if I change the IP address of PC B to 10.0.3.101. Will it then swap to the other VLAN?
  3. If I connect a PC instead of the switch, and I change the IP address as above, will it then/also "swap?"
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 13 '17 at 18:56
4

What an unmanaged switch will do with VLAN tagged frames is unpredictable. Some unmanaged switches will drop the tagged frames, while some may pass them on. Unmanaged switches which pass tagged frames will pass them on to the hosts connected to the switch, and most end-devices cannot use tagged frames, and they will be dropped.

In any case, frames from the hosts through the unmanaged switch will be placed on the trunk without VLAN tags, so all the frames will be part of the native VLAN, usually VLAN 1. This will place all the traffic in the same broadcast domain.

A host connected to a trunk will usually not work since the host will probably not understand VLAN tags. Some servers can, but most end-devices do not.

IP addresses have nothing to do with switches or VLANs, so changing IP addresses does not change VLANs. Switches and VLANs are layer-2, but IP addresses are layer-3.

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  • "IP addresses have nothing to do with switches or VLANs, so changing IP addresses does not change VLANs" - But if i change IP address of Device, will it change the VLAN the device is in? A device with the IP 10.0.1.100 will be in VLAN10 because 10.0.1.0/24 is assigned to VLAN10. If i change the ip address of this Device to 10.0.2.100, will it then be in VLAN20? Maybe VLAN10 is isolated from others and VLAN20 isn't. This would make the device reachable by all the others. So it would be a security risk. – SystemCookie May 31 '16 at 10:09
  • No. An IP address is a layer-3 address. A VLAN is a layer-2 broadcast domain. By changing the layer-3 address, you will not change its layer-2, but you will make it inoperable since the gateway for the new address is on a different layer-2 broadcast domain, and it can't reach it via layer-2. Hosts (routers or gateways are simply hosts to layer-2) on a VLAN communicate via layer-2 addressing, not layer-3 (IP) addressing. – Ron Maupin May 31 '16 at 10:23
  • And if I also change the Gateway? How can I change the VLAN my device is in? If a device is currently in VLAN10 and then I want it to be in VLAN20 because of reasons. – SystemCookie May 31 '16 at 10:26
  • 1
    You change the VLAN by changing it on the switch port. You don't seem to understand that layer-2 and layer-3 are independent of each other. Layer-2 can carry any number of layer-3 protocols (IPv4, IPX, IPv6, etc.) since it doesn't know or care about its payload. Layer-3 is also ignorant of the layer-2 protocol in which it is carried. IP is the payload of ethernet, token-ring, Wi-Fi, HDLC, PPP, etc., all of which are layer-2 protocols. VLANs are layer-2 broadcast domains. – Ron Maupin May 31 '16 at 10:36
  • VLANs are on managed switches. They are an artificial way of breaking a LAN into Virtual LANs. Trunks put VLAN tags into the layer-2 frame headers in order to distinguish frames for each VLAN so they can be separated again on the other end of the trunk. Which access port belongs to which VLAN is configured in the switch. It has nothing to do with any layer-3 addressing since switches are layer-2 devices, and they only read the layer-2 frame headers, not the frame payload (layer-3 packet). – Ron Maupin May 31 '16 at 10:44
1
  1. Depending on the switch, it won't be tagged at all and dropped or it will default to the default VLAN as stated above.

  2. As stated above, switching the IP has nothing to do with the VLANs, VLANs are layer 2 and the IP is layer 3. You can assigned whatever IP you want but if the packet is tagged with one of the 3 VLANs on the trunk then the traffic dies or falls to the default VLAN.

  3. If you just have the port configured as a trunk port with the 3 VLANs, the traffic from the PC will be dropped because it isn't tagged on any of the VLANs.

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  • Do you think your answer added anything that wasn't included in the answer above? – Ron Trunk May 31 '16 at 17:51
  • I think it directly answered his three questions. – Skyler Kincaid May 31 '16 at 18:37

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