User’s Manual of NS2503-24P/2C
68
- Tagging
- The act of putting 802.1Q VLAN information into the header of a packet.
- Untagging
- The act of stripping 802.1Q VLAN information out of the packet header.
802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address. Their
presence is indicated by a value of 0x8100 in the Ether Type field. When a packet's Ether Type field is equal to
0x8100
, the
packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3 bits of user priority,
1 bit of Canonical Format Identifier (CFI - used for encapsulating Token Ring 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
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 for tagged packets, tag-aware and
tag-unaware network devices can coexist on the same network.
A switch port can have only one PVID, but can have as many VID as the switch has memory in its VLAN table to store them.
Because some devices on a network may be tag-unaware, a decision must be made at each port on a tag-aware device before
packets are transmitted – should the packet to be transmitted have a tag or not? If the transmitting port is connected to a
tag-unaware device, the packet should be untagged. If the transmitting port is connected to a tag-aware device, the packet
should be tagged.
Original Ethernet
New Tagged Packet