Before understanding Virtual Local Area Networks (or VLANs), one should have a general understanding of the definition of a LAN.
A LAN consists of all devices in the same broadcast domain. VLANs create managed subsets of this domain, essentially seperating one broadcasting domain from another, in a virtual way. There is still one LAN, but multiple virtual LANs can be managed underneath this LAN. In other words, a first group of interfaces can be assigned to one VLAN, the second one assigned to another VLAN, etc.
The Cambium 450 platform is capable of working on the data link layer of the OSI model. It fully supports VLAN confugration and management, and even has a special feature called VLAN Remarking. The most convenient way of explaining this feature is a couple of examples. Let’s take, for instance, the uplink traffic. SM receives VLAN ID ‘X’ frame at the Ethernet interface, checks the configuration and re-marks to VLAN ID ‘Y’. So VLAN ID ‘Y’ frame comes out of AP’s Ethernet interface. When SM remarks, a dynamic entry in VLAN membership table for ‘Y’ is added to allow reception of VLAN ID ‘Y’ downstream packet. In the downlink traffic the opposite situation is taken place. AP receives VLAN ID ‘Y’ frame at the Ethernet interface and sends to SM. SM accepts the frame as it has an entry in the membership table and re-marks to VLAN ID ‘X’. This reverse remarking is necessary because the downstream devices do not know of remarking and are expecting VLAN ‘X’ frames. This remarking is done just before sending the packet out on Ethernet interface.
Hence, this remarking is completely invisible for devices below the SM and above the AP.
The official networking standard, that supports virtual LANs (VLANs) on an Ethernet network is IEEE 802.1Q. A special amendment to this standard called IEEE 802.1ad was created. The original 802.1Q specification only one VLAN header to be inserted into an Ethernet frame. QinQ allows multiple VLAN tags to be inserted into a single frame.
PMP450 SM supports remarking of the outer tag.