Interlogix IFS NS3550-24T/4S, User Manual

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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 46-1500 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, 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.
Original Ethernet
New Tagged Packet