Half of the ports of a switch are in vlan 10, the other half in vlan 20, all untagged. In some specific situations, we need someone in the field to be able to interconnect both vlans without access to (or knowledge of) the switch config. They will plug a cable from one of the ports in vlan 10 to one of the ports in vlan 20.
On an Alcatel Omniswitch 6250, we notice the interconnection is blocked by spanning tree, both in flat and in "1x1" mode. Since 1x1 mode runs a different STP instance for every VLAN, I was expecting the ports to keep forwarding traffic between the two vlans.
The result seems to be similar on a Juniper EX. In Wireshark, we see no difference between BPDUs from different vlans. More specifically, no vlan info is found in the bridge system ID field.
The obvious but suboptimal workaround is disabling STP on these ports, but am I wrong to assume this should be possible with per-vlan STP and/or MSTP?
Because someone might ask why we are doing this, the switch is used in a mobile setup to provide two networks with different types of uplinks (satellite and LTE). As a backup plan, the operator will sometimes need to be able to (easily and quickly) join the networks so all devices can use the available uplink.
This is basically a standalone switch, but since a physical (but not logical?) loop is sometimes created on purpose, we would like STP to step in when the operator mistakenly interconnects two ports in the same vlan.