Virtual LANs (VLANs)
BAT54-Rail/F..
Release
7.54
06/08
10.2
This is how a VLAN works
337
D VLAN ID: A unique number describes the virtual LAN. This ID defines the
belonging of data packets a logical (virtual) LAN. With this 12 bit value it
is possible to define up to 4094 different VLANs (VLAN IDs “0” and “4095”
are reserved resp. inadmissible).
Note: VLAN ID “1” is used by many devices as the Default VLAN ID. Con-
cerning unconfigured devices, all ports belong to this Default VLAN. How-
ever, this assignment can also be changed by configuration. (’The port
table’ → page 341).
D Priority: The priority of a VLAN-tagged data packet is indicated by a 3 bit
value. “0” represents the lowest priority, “7” the highest one. Data packets
without VLAN tag are treated with priority “0”.
This additional field makes the MAC frames longer than actually allowed.
These “overlong” packets can only be recognized and evaluated by VLAN-
capable stations and switches. Frame tagging incidentally leads to the de-
sired behavior for network users without VLAN support:
D Switches without VLAN support simply pass on these data packets and
ignore the additional fields within the MAC frame.
D Stations without VLAN support are not able to recognize the protocol type
due to the inserted VLAN tag and discard the packets silently.
Note: Older switches in the LAN are perhaps not able to pass on correctly
the overlong frames between the individual ports and will reject the
tagged packets.
10.2.2 Conversion within the LAN interconnection
Certain stations shall be grouped to logical units by virtual LANs. But the sta-
tions themselves are usually neither able to generate the required VLAN
tags, nor able to handle them.
Data traffic between network users always runs over different interfaces of
the distributors in the LAN. These distributors (switches, base stations) have
got the task to insert VLAN tags according to the desired application into the
data packets, to evaluate them and, if necessary, to remove them again. Be-
cause logical units are each connected to different interfaces of the distribu-
tors, the rules for generating and processing of the VLAN tags are assigned
to the single interfaces.
Coming back again to the first example:
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