VLAN tags enable a simple form of QoS control even when using switches that cannot evaluate IP headers. The IEEE 802.1p standard defines a priority tag in the VLAN header with a length of 3 bits, which correspond to the first 3 bits of the DSCP fields (Differentiated Services Code Point – DiffServ) and/or the precedence in the ToS field (Type of Service). The processing of VLAN tagged packets requires that packets in the receive direction are regarded differently to packets in the send direction.
- Upon receipt of a tagged Ethernet packet, it may be processed in one of three ways:
- The VLAN tag is ignored.
- The VLAN tag is always copied to the DiffServ or TOS field.
- The VLAN tag is copied to the DiffServ or TOS field if this is not marked already, i.e. the precedence is '000'.
- When a packet is transmitted over Ethernet, the VLAN tag can be set depending on the precedence. This should only happen if the recipient of the tag can understand it, i.e. tagged packets can be received. Tags are thus only set for packets which are sent to addresses from which the LCOS already received tagged packets.
Important: When a tagged packet is received, the tag is saved to the associated entry in the connection list. If a packet is to be sent with a precedence setting, then the VLAN ID recorded earlier is entered into the packet together with the precedence to form a VLAN tag. Where a connection causes other connections to be opened, e.g. with FTP, then the tag is inherited to the new entries.
