Planet WSW-2401A Инструкция по эксплуатации онлайн [44/95] 65301

packets so they can be carried across Ethernet backbones), and 12 bits of VLAN ID (VID). The 3 bits of user

priority are used by 802.1p. The VID is the VLAN identifier and is used by the 802.1Q standard. Because the VID

is 12 bits long, 4094 unique VLAN can be identified.

The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information

originally contained in the packet is retained.

802.1Q Tag

User Priority CFI VLAN ID (VID)

3 bits 1 bits 12 bits

TPID (Tag Protocol Identifier) TCI (Tag Control Information)

2 bytes 2 bytes

Preamble Destination

Address

Source

Address

VLAN TAG Ethernet

Type

Data FCS

6 bytes 6 bytes 4 bytes 2 bytes 46-1517 bytes 4 bytes

The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether

Type/Length or Logical Link Control. Because the packet is now a bit longer than it was originally, the Cyclic

Redundancy Check (CRC) must be recalculated.

Adding an IEEE802.1Q Tag

Dest. Addr. Src. Addr. Length/E. type Data Old CRC

Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC

Priority CFI VLAN ID

New Tagged Packet

Original Ethernet

Port VLAN ID

Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant

network device to another with the VLAN information intact. This allows 802.1Q VLAN to span network devices

(and indeed, the entire network – if all network devices are 802.1Q compliant).

Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch. If no

VLAN are defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1. Untagged

packets are assigned the PVID of the port on which they were received. Forwarding decisions are based upon this

PVID, in so far as VLAN are concerned. Tagged packets are forwarded according to the VID contained within the

tag. Tagged packets are also assigned a PVID, but the PVID is not used to make packet forwarding decisions, the

VID is.