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VLAN Overview
Topological
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
used instead. Routers are only needed to communicate between two VLANs.
Additional
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:
Destination
MAC Address
6
A VLAN tag consists of following fields:
Tag Protocol Identifier
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
User Priority
Canonical Format Indicator (CFI)
VLAN Identifier
6
independence: the use of VLANs allows non-physical grouping of nodes that share similar
improved: less routers, which are costly, are needed and VLAN-aware switches are
features: the VLAN user priority allows Layer 2 traffic separation.
VLAN tag
Source
TPID
TCI
MAC Address
6
2
2
Priority CFI
3
1
(TPID): this field is located at the same place as the Length/Type field of untagged
(TCI):
Field: this field indicates the 802.1p user priority.
(VID): this field identifies the VLAN to which the frame belongs.
Length/
Data/LLC
Type
2
0 .. 1500
Min. 46 bytes - Max. 1500 bytes
Min. 68 bytes - Max. 1522 bytes
VID
12
bits
Pad
FCS
46 .. 0
4
bytes
E-DOC-CTC-20080229-0001 v1.0
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