THOMSON R7.4, Configuration Manual

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E-DOC-CTC-20080229-0001 v1.0
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2| VLAN Overview
Topological independence : the use of VLANs allows non-physical grouping of nodes that share similar
resources. This results in the creation of logical networks, independent of the physical location of the
involved nodes. Hardware requirements are reduced, as networks can be separated logically instead of
physically.
Ease of administration : the management efforts to create (logical) networks are reduced:
Nodes can easily change their VLAN membership.
Network topology changes, for example a host moving to another location, no longer require
hardware changes but can be done via software.
Cost-effectiveness is improved : less routers, which are costly, are needed and VLAN-aware switches are
used instead. Routers are only needed to communicate between two VLANs.
Additional features : the VLAN user priority allows Layer 2 traffic separation.
VLAN-tagged frame format
In order to support VLANs, the Ethernet frame format is extended with a VLAN tag, indicated in red in the
following illustration:
A VLAN tag consists of following fields:
Tag Protocol Identifier (TPID) : this field is located at the same place as the Length/Type field of untagged
frames. The value of the field is always equal to 0x8100, which indicates that the frame is tagged and that
the original Length/Type field is located after the VLAN tag.
Tag Control Information (TCI) :
User Priority Field : this field indicates the 802.1p user priority.
Canonical Format Indicator (CFI)
VLAN Identifier (VID) : this field identifies the VLAN to which the frame belongs.
Destination
MAC Address
Source
MAC Address
Length/
Type
TPID TCI FCS
6 6 2 4 2 2
Pad Data/LLC
0 .. 1500 46 .. 0 bytes
Min. 46 bytes - Max. 1500 bytes
Min. 68 bytes - Max. 1522 bytes
Priority CFI VID
3 1 12 bits
VLAN tag