Virtual LANs (VLANs)
BAT54-Rail/F..
Release
7.54
06/08
10.6
VLAN tags on layer 2/3 in the Ethernet
347
10.6VLAN tags on layer 2/3 in the
Ethernet
VLAN tags enable a simple form of QoS control even when using switches
that cannot evaluate IP headers. The IEEE 802.1p standard defines a priority
tag in the VLAN header with a length of 3 bits, which correspond to the first
3 bits of the DSCP fields (Differentiated Services Code Point - DiffServ) and/
or the precedence in the TOS field (Type of Service). The processing of
VLAN tagged packets requires that packets in the receive direction are re-
garded differently to packets in the send direction.
D Upon receipt of a tagged Ethernet packet, it may be processed in one of
three ways:
D The VLAN tag is ignored.
D The VLAN tag is always copied to the DiffServ or TOS field.
D The VLAN tag is copied to the DiffServ or TOS field if this is not marked
already, i.e. the precedence is '000'.
D When a packet is transmitted over Ethernet, the VLAN tag can be set de-
pending on the precedence. This should only happen if the recipient of
the tag can understand it, i.e. tagged packets can be received. Tags are
thus only set for packets which are sent to addresses from which the BAT
already received tagged packets.
Note: When a tagged packet is received, the tag is saved to the associated
entry in the connection list. If a packet is to be sent with a precedence set-
ting, then the VLAN ID recorded earlier is entered into the packet together
with the precedence to form a VLAN tag. Where a connection causes oth-
er connections to be opened, e.g. with FTP or H.323, then the tag is in-
herited to the new entries.
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