I have an interesting issue. I have to use a switch to dynamically connect and disconnect multiple devices together in a point to point configuration, without changing anything on the end user side.

For example I would first like computer A and B to be point to point, then after some time I would like A and C to be point to point. ETC. I am not worried about the changing I have automation scripts that can handle it. The problem is the computers are running their own randomized vlans, and I do not want to know or care what vlan they are on, as it is a range. I am using an fs S3900-48T4S.

diagram of the situation


This switch has many features and one is QinQ which I thought would be perfect so I set the configuration like this:

interface ethernet 1/8
switchport mode access
switchport access vlan 2
switchport dot1q-tunnel mode access

Then the same thing on side b

interface ethernet 1/9
switchport mode access
switchport access vlan 2
switchport dot1q-tunnel mode access

this kinda works but it causes toggling in the connection. One minute it will be connected the next not connected.

Is there another way to do this, or am I missing a setting.


  • 1
    Ethernet is not a p-t-p technology, but I think the q-in-q process is the right path. Unless you want to build some hardware...
    – Ricky
    May 26, 2020 at 20:13
  • Can you expand on your description of toggling in the connection? Is the layer-1 link between the switch and the PCs going down; or is the Ethernet link reporting UP but traffic will not pass between the PCs? Does the switch log or CLI yield any clues? How about tcpdump or wireshark? May 26, 2020 at 20:57
  • The two devices are actually networking devices that are being tested. On them they can tell that they are connected through a cli prompt. For example on node a it will say it is connected to node b on vlan x, then after waiting 15 seconds or so it will say its not connected anymore. The switch can handle port mirroring so wireshark is probably the next course of action, when corona allows me in the lab. I was just wondering if I was going down a bad path. Thanks for the help! May 27, 2020 at 0:36


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