Allied Telesis 2.6.1 manual Format of user priority and Vlan data in an Ethernet frame

74 AT-8800 Series Switch User Guide

Figure 13: Format of user priority and VLAN data in an Ethernet frame.

	Destination	Source	Type/
Preamble	Address	Address	Length	Frame Data	CRC
64 bits	48 bits	48 bits	16	368-12000	32 bits
			bits
	TPID	User	CFI	VID
		Priority
	16 bits	3 bits	bit	12 bits
			1
	0x81-00				SWITCH6

Table 11: Reserved VID values .

VID value (hexadecimal) Meaning and use of reserved VID values

0	The null VLAN ID. Indicates that the tag header contains only
	user priority information; no VLAN Identifier is present in the
	frame. This VID value must not be configured in any Forwarding
	Database entry, or used in any management operation. Frames
	that contain the null VLAN ID are also known as priority-tagged
	frames.
1	The default VID value used for classifying frames on ingress
	through an untagged switch port.
FFF	Reserved for implementation use. This VID value must not be
	configured in any Forwarding Database entry, used in any
	management operation, or transmitted in a tag header.

Ethernet packets which contain a VLAN tag are referred to as tagged frames, and switch ports that transmit tagged frames are referred to as tagged ports. Ethernet packets which do not contain the VLAN tag are referred to as untagged frames, and switch ports that transmit untagged frames are referred to as untagged ports. VLANs can consist of simple logical groupings of untagged ports, in which the ports receive and transmit untagged packets. Alternatively, VLANs can contain only tagged ports, or a mixture of tagged and untagged ports.

The switch is VLAN aware. It can accept VLAN tagged frames, and supports the VLAN switching required by such tags. A network can contain a mixture of VLAN aware devices, for example, other 802.1Q-compatible switches, and VLAN unaware devices, for example, workstations and legacy switches that do not support VLAN tagging. The switch can be configured to send VLAN tagged or untagged frames on each port, depending on whether or not the devices connected to the port are VLAN aware. By assigning a port to two different VLANs, to one as an untagged port and to another as a tagged port, it is possible for the port to transmit both VLAN-tagged and untagged frames. A port must belong to a VLAN at all times unless the port has been set as the mirror port for the switch.

Software Release 2.6.1 C613-02039-00 REV A