Chapter 4: Web-Based Management

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

Source

 

 

VLAN TAG

 

Ethernet

 

Data

FCS

Address

Address

 

 

 

Type

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

Original Ethernet

Dest. Addr.

Src. Addr.

E. type

Tag

Length/E. type

Data

New CRC

New Tagged Packet

Priority

CFI

VLAN ID

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.

Tag-aware switches must keep a table to relate PVID within the switch to VID on the network. The switch will compare the VID of a packet to be transmitted to the VID of the port that is to transmit the packet. If the two VID are different the switch will drop the packet. Because of the existence of the PVID for untagged packets and the VID

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GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual

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GE GE-DS-82, 82-POE user manual 802.1Q Tag