Hello Giovanni
General mode is indeed available on switches of other vendors as well, including the Cisco small business series of switches. General mode causes ports to function differently for egress and ingress traffic.
- For egress, general mode allows a switch to send untagged frames out of an interface, that come from multiple VLANs.
- For ingress, all untagged frames must go to the configured native VLAN. This is because, regardless of which VLAN these frames originated from, there is no way to distinguish to which VLAN they belong since they have no tags. By definition, the configured native VLAN is the VLAN on which all incoming untagged frames must be placed.
This means that untagged frames originating from multiple VLANs on SW1 will all end up on the configured native VLAN of the port on SW2.
Now to be honest, after doing some additional research, I find no obvious benefit to the general mode for an interface. There may be an obscure use case where it is beneficial, but I haven’t found anything myself. Cisco IOS doesn’t support it, but the small office devices do.
I hope this has been helpful!
Laz