Interlogix GE-DS-242-PoE - Manual (Page 61)

Chapter 4: Console Management
GE-DS-242-PoE Managed Ethernet Switch User Manual 57
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
Original Ethernet
New Tagged Packet