Interlogix MC355-1T/1S User Manual - Manual (Page 40)

36 IFS MC355-1T/1S User Manual
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 46-1500 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.
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
Original Ethernet
New Tagged Packet