Allied Telesis 2.6.1 manual Vlan Tagging, Tpid

Layer 2 Switching 73

Devices that are members of the same VLAN only exchange data with each other through the switch’s switching capabilities. To exchange data between devices in separate VLANs, the switch’s routing capabilities are used. The switch passes VLAN status information, indicating whether a VLAN is up or down, to the Internet Protocol (IP) module. IP uses this information to determine route availability.

The switch has a maximum of 255 VLANs, ranging from a VLAN identifier (VID) of 1 to 4094. When the switch is first powered up, a “default” VLAN is created and all ports are added to it. In this initial unconfigured state, the switch will broadcast all the packets it receives to the default VLAN. This VLAN has a VID of 1 and an interface name of vlan1. It cannot be deleted, and ports can only be removed from it if they also belong to at least one other VLAN. The default VLAN cannot be added to any STP, but always belongs to the default STP. If all the devices on the physical LAN are to belong to the same logical LAN, that is, the same broadcast domain, then the default settings will be acceptable, and no additional VLAN configuration is required.

VLAN Tagging

An Ethernet packet can contain a VLAN tag, with fields that specify VLAN membership and user priority. The VLAN tag is described in IEEE Standard 802.3ac, and is four octets that can be inserted between the Source Address and the Type/Length fields in the Ethernet packet (Figure 13 on page 74). To accommodate the tag, Standard 802.3ac also increased the maximum allowable length for an Ethernet frame to 1522 octets (the minimum size is 64 octets). IEEE Standard 802.1Q specifies how the data in the VLAN tag is used to switch frames. VLAN-aware devices are able to add the VLAN tag to the packet header. VLAN-unaware devices cannot set or read the VLAN tag.

Table 10 on page 73 lists the meaning and use of the fields in the Ethernet frame. Figure 13 on page 74 shows the format of VLAN data in an Ethernet frame. Twelve bits of the tag are the VLAN Identifier (VID), which indicates the VLAN that the packet belongs to. Table 11 on page 74 lists the VLAN Identifier values that have specific meaning.

Table 10: Fields in the Ethernet frame for QoS and VLAN switching.

Field	Length	Meaning and use
TPID	2 octets	The Tag Protocol Identifier (TPID) is defined by IEEE Standard
		802.1Q as 0x81-00.
User Priority	3 bits	The User Priority field is the priority tag for the frame, which
		can be used by the switch to determine the Quality of
		Service to apply to the frame. The three bit binary number
		represents eight priority levels, 0 to 7.
CFI	1 bit	The Canonical Format Indicator (CFI flag) is used to indicate
		whether all MAC address information that may be present
		in the MAC data carried by the frame is in canonical format.
VID	12 bits	The VLAN Identifier (VID) field uniquely identifies the VLAN
		to which the frame belongs.

Software Release 2.6.1 C613-02039-00 REV A