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Chapter 4: Web-Based Management
802.1Q Tag
TPID (Tag Protocol Identifier)
Destination
Preamble
Address
6 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.
Dest. Addr.
Src. Addr.
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
56
User Priority
3 bits
TCI (Tag Control Information)
2 bytes
Source
VLAN TAG
Address
6 bytes
4 bytes
Length/E. type
Data
Length/E. type
E. type
Tag
Priority
CFI
VLAN ID
CFI
VLAN ID (VID)
1 bits
12 bits
2 bytes
Ethernet
Data
Type
2 bytes
46-1517 bytes
Original Ethernet
Old CRC
Data
New CRC
GE-DS-242-PoE Managed Ethernet Switch User Manual
FCS
4 bytes
New Tagged Packet
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