I have VMs connected to a single NIC port, and that physical port is connected to a physical switch port.

|   Server    |
|+----+ +----+|
||VM_A| |VM_B||
|+--+-+ +-+--+|
|   +--+--+   |  <- Here is a virtual switch (*)

Assuming that the virtual switch (*) in the server can ...

  • (downwards traffic) just transport traffic from VMs to the L2 switch being unaware of L2
  • (upwards traffic) classify packets to each VMs using its static MAC address table set

are there any ways to configure the physical L2 switch to enable VM_A and VM_B to communicate?

i.e.) I want the L2 switch to send the packets back to the port from which the packet just came

e.g.) Having two static entries in the MAC address tables on the physical L2 switch supports that kind of loopback?

$ mac address-table static <MAC_VM_A> vlan 1 interface <IF0>
$ mac address-table static <MAC_VM_B> vlan 1 interface <IF0>

I have CRS326 and CSS326 switches and can consider other switches, too.

  • 1
    An actual layer-2 switch will not send a frame (not a packet) back out the interface on which the frame was received (that could result in layer-2 loops that crash the LAN). Your virtual switch should send the frame between the VMs, but what happens inside your VM server is off-topic here. – Ron Maupin Dec 14 '19 at 6:49
  • 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 can post and accept your own answer. – Ron Maupin Dec 17 '20 at 14:31

Short answer: no.

A switch will never forward a frame back to its ingress port. If that port was used to connect a repeater hub it would be quite normal to receive frames with destination MACs associated with the very same port. The switch doesn't need to bridge them to any other port, so they are simply dropped. Whether the MAC/port association has been learned automatically or configured manually doesn't matter.

Even if the switch wouldn't know the destination MAC yet, it'd flood the frame to all other ports but the one it was received from, mimicking a repeater. Any other logic would created endless bridge loops, bringing down the network.

Instead, it's the virtual switch's job to bridge frames between virtual NICs/ports. The physical switch can't do that.

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