it should be a vlan 0 and not vlan 1? I cannot seem to understand the difference, perhaps there is a mikrotik document explaining it?
Mikrotik documentation rarely explains standards or chip manufacturer documentation.
In 802.1Q, VID 0 means "treat the frame as it was taggless except that you respect the priority field of the tag" - the tag has a fixed format so 0 is the way to say the VID part of it is "unused". That's why one of the values of the
admit-frames parameter of
/interface bridge port row is
admit-only-untagged-and-priority-tagged.
Similarly, VID 4095 (all 1s) seems to have some reserved meaning, various vendors treat it differently.
The fact that the 8227 switch chip accepts 0 as a valid VID when it comes to indicating which port belongs to which VLAN, as specified in the
/interface ethernet switch vlan branch of the configuration tree, seems rather like an anomaly/side effect than an intention to me.
VID 1 is just a regular VID like any other in the 1-4094 range, except many manufacturers make it the "native VLAN" of hybrid ports, which means it is not shown in default configurations. And thanks to their poor understanding, many people think VLAN 1 is some kind of black magic and avoid treating it as a VLAN.
Is this behavior relevant to RouterOS in general or only for a small subset of switch chips?
I had no reason to try this on any other switch chip than the 8227 as all the other chips either do not support VLAN filtering in hardware at all (at least in RouterOS 6, some do in RouterOS 7) or hybrid ports can be configured on them without crazy workarounds.